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Cells in a Range on an Excel Sheet

cells-in-a-range-on-an-excel-sheet

A cell (r, c) of an excel sheet is represented as a string "<col><row>" where: You are given a string s in the format "<col1><row1>:<col2><row2>", where <col1> represents the column c1, <row1> represents the row r1, <col2> represents the column c2, and <row2> represents the row r2, such that r1 <= r2 and c1 <= c2. Return the list of cells (x, y) such that r1 <= x <= r2 and c1 <= y <= c2. The cells should be represented as strings in the format mentioned above and be sorted in non-decreasing order first by columns and then by rows.

String

Easy

From the given string, find the corresponding rows and columns. Iterate through the columns in ascending order and for each column, iterate through the rows in ascending order to obtain the required cells in sorted order.

168,171,1094

Append K Integers With Minimal Sum

append-k-integers-with-minimal-sum

You are given an integer array nums and an integer k. Append k unique positive integers that do not appear in nums to nums such that the resulting total sum is minimum. Return the sum of the k integers appended to nums.

Array,Math,Greedy,Sorting

Medium

The k smallest numbers that do not appear in nums will result in the minimum sum. Recall that the sum of the first n positive numbers is equal to n * (n+1) / 2. Initialize the answer as the sum of 1 to k. Then, adjust the answer depending on the values in nums.

402,448,1646

Create Binary Tree From Descriptions

create-binary-tree-from-descriptions

You are given a 2D integer array descriptions where descriptions[i] = [parenti, childi, isLefti] indicates that parenti is the parent of childi in a binary tree of unique values. Furthermore, Construct the binary tree described by descriptions and return its root. The test cases will be generated such that the binary tree is valid.

Array,Hash Table,Tree,Depth-First Search,Breadth-First Search,Binary Tree

Medium

Could you represent and store the descriptions more efficiently? Could you find the root node? The node that is not a child in any of the descriptions is the root node.

109,1820

Replace Non-Coprime Numbers in Array

replace-non-coprime-numbers-in-array

You are given an array of integers nums. Perform the following steps: Return the final modified array. It can be shown that replacing adjacent non-coprime numbers in any arbitrary order will lead to the same result. The test cases are generated such that the values in the final array are less than or equal to 108. Two values x and y are non-coprime if GCD(x, y) > 1 where GCD(x, y) is the Greatest Common Divisor of x and y.

Array,Math,Stack,Number Theory

Hard

Notice that the order of merging two numbers into their LCM does not matter so we can greedily merge elements to its left if possible. If a new value is formed, we should recursively check if it can be merged with the value to its left. To simulate the merge efficiently, we can maintain a stack that stores processed elements. When we iterate through the array, we only compare with the top of the stack (which is the value to its left).

1320,2129

Divide Array Into Equal Pairs

divide-array-into-equal-pairs

You are given an integer array nums consisting of 2 * n integers. You need to divide nums into n pairs such that: Return true if nums can be divided into n pairs, otherwise return false.

Array,Hash Table,Bit Manipulation,Counting

Easy

For any number x in the range [1, 500], count the number of elements in nums whose values are equal to x. The elements with equal value can be divided completely into pairs if and only if their count is even.

1741

Maximize Number of Subsequences in a String

maximize-number-of-subsequences-in-a-string

You are given a 0-indexed string text and another 0-indexed string pattern of length 2, both of which consist of only lowercase English letters. You can add either pattern[0] or pattern[1] anywhere in text exactly once. Note that the character can be added even at the beginning or at the end of text. Return the maximum number of times pattern can occur as a subsequence of the modified text. A subsequence is a string that can be derived from another string by deleting some or no characters without changing the order of the remaining characters.

String,Greedy,Prefix Sum

Medium

Find the optimal position to add pattern[0] so that the number of subsequences is maximized. Similarly, find the optimal position to add pattern[1]. For each of the above cases, count the number of times the pattern occurs as a subsequence in text. The larger count is the required answer.

1250

Minimum Operations to Halve Array Sum

minimum-operations-to-halve-array-sum

You are given an array nums of positive integers. In one operation, you can choose any number from nums and reduce it to exactly half the number. (Note that you may choose this reduced number in future operations.) Return the minimum number of operations to reduce the sum of nums by at least half.

Array,Greedy,Heap (Priority Queue)

Medium

It is always optimal to halve the largest element. What data structure allows for an efficient query of the maximum element? Use a heap or priority queue to maintain the current elements.

2094

Minimum White Tiles After Covering With Carpets

minimum-white-tiles-after-covering-with-carpets

You are given a 0-indexed binary string floor, which represents the colors of tiles on a floor: You are also given numCarpets and carpetLen. You have numCarpets black carpets, each of length carpetLen tiles. Cover the tiles with the given carpets such that the number of white tiles still visible is minimum. Carpets may overlap one another. Return the minimum number of white tiles still visible.

String,Dynamic Programming,Prefix Sum

Hard

Can you think of a DP solution? Let DP[i][j] denote the minimum number of white tiles still visible from indices i to floor.length-1 after covering with at most j carpets. The transition will be whether to put down the carpet at position i (if possible), or not.

72

Most Frequent Number Following Key In an Array

most-frequent-number-following-key-in-an-array

You are given a 0-indexed integer array nums. You are also given an integer key, which is present in nums. For every unique integer target in nums, count the number of times target immediately follows an occurrence of key in nums. In other words, count the number of indices i such that: Return the target with the maximum count. The test cases will be generated such that the target with maximum count is unique.

Array,Hash Table,Counting

Easy

Count the number of times each target value follows the key in the array. Choose the target with the maximum count and return it.

1741

Longest Winning Streak

longest-winning-streak

Database

Hard

Remove All Ones With Row and Column Flips II

remove-all-ones-with-row-and-column-flips-ii

Array,Bit Manipulation,Breadth-First Search,Matrix

Medium

With the given constraints, could a brute force solution pass? What would a brute force solution look like? We can try every single possibility of choosing to do an operation on a cell with a 1 or choosing to ignore it.

73,1409,2268

The Change in Global Rankings

the-change-in-global-rankings

Database

Medium

Count Hills and Valleys in an Array

count-hills-and-valleys-in-an-array

You are given a 0-indexed integer array nums. An index i is part of a hill in nums if the closest non-equal neighbors of i are smaller than nums[i]. Similarly, an index i is part of a valley in nums if the closest non-equal neighbors of i are larger than nums[i]. Adjacent indices i and j are part of the same hill or valley if nums[i] == nums[j]. Note that for an index to be part of a hill or valley, it must have a non-equal neighbor on both the left and right of the index. Return the number of hills and valleys in nums.

Array

Easy

For each index, could you find the closest non-equal neighbors? Ensure that adjacent indices that are part of the same hill or valley are not double-counted.

162,932,1519

Count Collisions on a Road

count-collisions-on-a-road

There are n cars on an infinitely long road. The cars are numbered from 0 to n - 1 from left to right and each car is present at a unique point. You are given a 0-indexed string directions of length n. directions[i] can be either 'L', 'R', or 'S' denoting whether the ith car is moving towards the left, towards the right, or staying at its current point respectively. Each moving car has the same speed. The number of collisions can be calculated as follows: After a collision, the cars involved can no longer move and will stay at the point where they collided. Other than that, cars cannot change their state or direction of motion. Return the total number of collisions that will happen on the road.

String,Stack

Medium

In what circumstances does a moving car not collide with another car? If we disregard the moving cars that do not collide with another car, what does each moving car contribute to the answer? Will stationary cars contribute towards the answer?

735,883,1627,1902

Maximum Points in an Archery Competition

maximum-points-in-an-archery-competition

Alice and Bob are opponents in an archery competition. The competition has set the following rules: For example, if Alice and Bob both shot 2 arrows on the section with score 11, then Alice takes 11 points. On the other hand, if Alice shot 0 arrows on the section with score 11 and Bob shot 2 arrows on that same section, then Bob takes 11 points. You are given the integer numArrows and an integer array aliceArrows of size 12, which represents the number of arrows Alice shot on each scoring section from 0 to 11. Now, Bob wants to maximize the total number of points he can obtain. Return the array bobArrows which represents the number of arrows Bob shot on each scoring section from 0 to 11. The sum of the values in bobArrows should equal numArrows. If there are multiple ways for Bob to earn the maximum total points, return any one of them.

Array,Bit Manipulation,Recursion,Enumeration

Medium

To obtain points for some certain section x, what is the minimum number of arrows Bob must shoot? Given the small number of sections, can we brute force which sections Bob wants to win? For every set of sections Bob wants to win, check if we have the required amount of arrows. If we do, it is a valid selection.

2130

Longest Substring of One Repeating Character

longest-substring-of-one-repeating-character

You are given a 0-indexed string s. You are also given a 0-indexed string queryCharacters of length k and a 0-indexed array of integer indices queryIndices of length k, both of which are used to describe k queries. The ith query updates the character in s at index queryIndices[i] to the character queryCharacters[i]. Return an array lengths of length k where lengths[i] is the length of the longest substring of s consisting of only one repeating character after the ith query is performed.

Array,String,Segment Tree,Ordered Set

Hard

Use a segment tree to perform fast point updates and range queries. We need each segment tree node to store the length of the longest substring of that segment consisting of only 1 repeating character. We will also have each segment tree node store the leftmost and rightmost character of the segment, the max length of a prefix substring consisting of only 1 repeating character, and the max length of a suffix substring consisting of only 1 repeating character. Use this information to properly merge the two segment tree nodes together.

56,424,1542,1772

Find All K-Distant Indices in an Array

find-all-k-distant-indices-in-an-array

You are given a 0-indexed integer array nums and two integers key and k. A k-distant index is an index i of nums for which there exists at least one index j such that |i - j| <= k and nums[j] == key. Return a list of all k-distant indices sorted in increasing order.

Array

Easy

For every occurrence of key in nums, find all indices within distance k from it. Use a hash table to remove duplicate indices.

1,243

Minimum Weighted Subgraph With the Required Paths

minimum-weighted-subgraph-with-the-required-paths

You are given an integer n denoting the number of nodes of a weighted directed graph. The nodes are numbered from 0 to n - 1. You are also given a 2D integer array edges where edges[i] = [fromi, toi, weighti] denotes that there exists a directed edge from fromi to toi with weight weighti. Lastly, you are given three distinct integers src1, src2, and dest denoting three distinct nodes of the graph. Return the minimum weight of a subgraph of the graph such that it is possible to reach dest from both src1 and src2 via a set of edges of this subgraph. In case such a subgraph does not exist, return -1. A subgraph is a graph whose vertices and edges are subsets of the original graph. The weight of a subgraph is the sum of weights of its constituent edges.

Graph,Shortest Path

Hard

Consider what the paths from src1 to dest and src2 to dest would look like in the optimal solution. It can be shown that in an optimal solution, the two paths from src1 and src2 will coincide at one node, and the remaining part to dest will be the same for both paths. Now consider how to find the node where the paths will coincide. How can algorithms for finding the shortest path between two nodes help us?

1485

Number of Ways to Build Sturdy Brick Wall

number-of-ways-to-build-sturdy-brick-wall

Array,Dynamic Programming,Bit Manipulation,Bitmask

Medium

A row of bricks can be represented uniquely by the points where two bricks are joined together. For a given row of bricks, how many configurations of bricks could you have put below this row such that the wall is sturdy? Use dynamic programming to store the number of possible sturdy walls with a given height and configuration of bricks on the top row.

554,821

Minimum Bit Flips to Convert Number

minimum-bit-flips-to-convert-number

A bit flip of a number x is choosing a bit in the binary representation of x and flipping it from either 0 to 1 or 1 to 0. Given two integers start and goal, return the minimum number of bit flips to convert start to goal.

Bit Manipulation

Easy

If the value of a bit in start and goal differ, then we need to flip that bit. Consider using the XOR operation to determine which bits need a bit flip.

1441

Find Triangular Sum of an Array

find-triangular-sum-of-an-array

You are given a 0-indexed integer array nums, where nums[i] is a digit between 0 and 9 (inclusive). The triangular sum of nums is the value of the only element present in nums after the following process terminates: Return the triangular sum of nums.

Array,Math,Simulation,Combinatorics

Medium

Try simulating the entire process. To reduce space, use a temporary array to update nums in every step instead of creating a new array at each step.

119

Number of Ways to Select Buildings

number-of-ways-to-select-buildings

You are given a 0-indexed binary string s which represents the types of buildings along a street where: As a city official, you would like to select 3 buildings for random inspection. However, to ensure variety, no two consecutive buildings out of the selected buildings can be of the same type. Return the number of valid ways to select 3 buildings.

String,Dynamic Programming,Prefix Sum

Medium

There are only 2 valid patterns: ‘101’ and ‘010’. Think about how we can construct these 2 patterns from smaller patterns. Count the number of subsequences of the form ‘01’ or ‘10’ first. Let n01[i] be the number of ‘01’ subsequences that exist in the prefix of s up to the ith building. How can you compute n01[i]? Let n0[i] and n1[i] be the number of ‘0’s and ‘1’s that exists in the prefix of s up to i respectively. Then n01[i] = n01[i – 1] if s[i] == ‘0’, otherwise n01[i] = n01[i – 1] + n0[i – 1]. The same logic applies to building the n10 array and subsequently the n101 and n010 arrays for the number of ‘101’ and ‘010‘ subsequences.

Sum of Scores of Built Strings

sum-of-scores-of-built-strings

You are building a string s of length n one character at a time, prepending each new character to the front of the string. The strings are labeled from 1 to n, where the string with length i is labeled si. The score of si is the length of the longest common prefix between si and sn (Note that s == sn). Given the final string s, return the sum of the score of every si.

String,Binary Search,Rolling Hash,Suffix Array,String Matching,Hash Function

Hard

Each s_i is a suffix of the string s, so consider algorithms that can determine the longest prefix that is also a suffix. Could you use the Z array from the Z algorithm to find the score of each s_i?

1508

Largest Number After Digit Swaps by Parity

largest-number-after-digit-swaps-by-parity

You are given a positive integer num. You may swap any two digits of num that have the same parity (i.e. both odd digits or both even digits). Return the largest possible value of num after any number of swaps.

Sorting,Heap (Priority Queue)

Easy

The bigger digit should appear first (more to the left) because it contributes more to the value of the number. Get all the even digits, as well as odd digits. Sort them separately. Reconstruct the number by giving the earlier digits the highest available digit of the same parity.

748,941,958,1308,2271

Minimize Result by Adding Parentheses to Expression

minimize-result-by-adding-parentheses-to-expression

You are given a 0-indexed string expression of the form "<num1>+<num2>" where <num1> and <num2> represent positive integers. Add a pair of parentheses to expression such that after the addition of parentheses, expression is a valid mathematical expression and evaluates to the smallest possible value. The left parenthesis must be added to the left of '+' and the right parenthesis must be added to the right of '+'. Return expression after adding a pair of parentheses such that expression evaluates to the smallest possible value. If there are multiple answers that yield the same result, return any of them. The input has been generated such that the original value of expression, and the value of expression after adding any pair of parentheses that meets the requirements fits within a signed 32-bit integer.

String,Enumeration

Medium

The maximum length of expression is very low. We can try every possible spot to place the parentheses. Every possibility of expression is of the form a * (b + c) * d where a, b, c, d represent integers. Note the edge cases where a and/or d do not exist, in which case use 1 instead of them.

224,241,640

Maximum Product After K Increments

maximum-product-after-k-increments

You are given an array of non-negative integers nums and an integer k. In one operation, you may choose any element from nums and increment it by 1. Return the maximum product of nums after at most k operations. Since the answer may be very large, return it modulo 109 + 7. Note that you should maximize the product before taking the modulo.

Array,Greedy,Heap (Priority Queue)

Medium

If you can increment only once, which number should you increment? We should always prioritize the smallest number. What kind of data structure could we use? Use a min heap to hold all the numbers. Each time we do an operation, replace the top of the heap x by x + 1.

209,982,1938

Maximum Total Beauty of the Gardens

maximum-total-beauty-of-the-gardens

Alice is a caretaker of n gardens and she wants to plant flowers to maximize the total beauty of all her gardens. You are given a 0-indexed integer array flowers of size n, where flowers[i] is the number of flowers already planted in the ith garden. Flowers that are already planted cannot be removed. You are then given another integer newFlowers, which is the maximum number of flowers that Alice can additionally plant. You are also given the integers target, full, and partial. A garden is considered complete if it has at least target flowers. The total beauty of the gardens is then determined as the sum of the following: Return the maximum total beauty that Alice can obtain after planting at most newFlowers flowers.

Array,Two Pointers,Binary Search,Greedy,Sorting

Hard

Say we choose k gardens to be complete, is there an optimal way of choosing which gardens to plant more flowers to achieve this? For a given k, we should greedily fill-up the k gardens with the most flowers planted already. This gives us the most remaining flowers to fill up the other gardens. After sorting flowers, we can thus try every possible k and what is left is to find the highest minimum flowers we can obtain by planting the remaining flowers in the other gardens. To find the highest minimum in the other gardens, we can use binary search to find the most optimal way of planting.

410

Finding the Topic of Each Post

finding-the-topic-of-each-post

Database

Hard

1625

The Number of Users That Are Eligible for Discount

the-number-of-users-that-are-eligible-for-discount

Database

Easy

177,2371

Minimum Number of Operations to Convert Time

minimum-number-of-operations-to-convert-time

You are given two strings current and correct representing two 24-hour times. 24-hour times are formatted as "HH:MM", where HH is between 00 and 23, and MM is between 00 and 59. The earliest 24-hour time is 00:00, and the latest is 23:59. In one operation you can increase the time current by 1, 5, 15, or 60 minutes. You can perform this operation any number of times. Return the minimum number of operations needed to convert current to correct.

String,Greedy

Easy

Convert the times to minutes. Use the operation with the biggest value possible at each step.

322

Minimum Health to Beat Game

minimum-health-to-beat-game

Array,Greedy,Prefix Sum

Medium

When should you use your armor ability? It is always optimal to use your armor ability on the level where you take the most amount of damage.

174,2049

Maximum Sum Score of Array

maximum-sum-score-of-array

Array,Prefix Sum

Medium

How can we use precalculation to efficiently calculate the average difference at an index? Create a prefix and/or suffix sum array.

560,724,2102

Users With Two Purchases Within Seven Days

users-with-two-purchases-within-seven-days

Database

Medium

1852

The Users That Are Eligible for Discount

the-users-that-are-eligible-for-discount

Database

Easy

2336

Number of Times a Driver Was a Passenger

number-of-times-a-driver-was-a-passenger

Medium

1779,1785,1795

Add Two Integers

add-two-integers

Math

Easy

Root Equals Sum of Children

root-equals-sum-of-children

You are given the root of a binary tree that consists of exactly 3 nodes: the root, its left child, and its right child. Return true if the value of the root is equal to the sum of the values of its two children, or false otherwise.

Tree,Binary Tree

Easy

Count Positions on Street With Required Brightness

count-positions-on-street-with-required-brightness

Medium

How can we find the brightness at every position on the street? We can use a hash table to store the change in brightness from the previous position to the current position.

370,2075