Grind75

Question 1

Two Sum

Answer 1

Use a hash map to store the difference of the target and the current element as the key and the index as the value.

Question 2

Best Time To Buy And Sell Stock,https://leetcode.com/problems/best-time-to-buy-and-sell-stock

Answer 2

Keep track of the minimum price and maximum profit while iterating through the list.

Question 3

Majority Element,https://leetcode.com/problems/majority-element

Answer 3

Use Boyer-Moore Voting Algorithm to find the majority element.

Question 4

Contains Duplicate,https://leetcode.com/problems/contains-duplicate

Answer 4

Use a set to check for duplicates.

Question 5

Insert Interval,https://leetcode.com/problems/insert-interval

Answer 5

Merge intervals while iterating through the list.

Question 6

3Sum,https://leetcode.com/problems/3sum

Answer 6

Sort the array and use three pointers to find the triplets.

Question 7

Product Of Array Except Self,https://leetcode.com/problems/product-of-array-except-self

Answer 7

Use two arrays to store the product of all elements to the left and right of each element.

Question 8

Combination Sum,https://leetcode.com/problems/combination-sum

Answer 8

Use backtracking to find all possible combinations.

Question 9

Merge Intervals,https://leetcode.com/problems/merge-intervals

Answer 9

Sort intervals by starting point and merge overlapping intervals.

Question 10

Sort Colors,https://leetcode.com/problems/sort-colors

Answer 10

Use the Dutch National Flag algorithm to sort the colors in one pass.

Question 11

Container With Most Water,https://leetcode.com/problems/container-with-most-water

Answer 11

Use two pointers to find the maximum area.

Question 12

Invert Binary Tree,https://leetcode.com/problems/invert-binary-tree

Answer 12

Use a recursive function to swap the left and right children of each node.

Question 13

Balanced Binary Tree,https://leetcode.com/problems/balanced-binary-tree

Answer 13

Check if the difference in heights of the left and right subtrees is no more than one.

Question 14

Diameter of Binary Tree,https://leetcode.com/problems/diameter-of-binary-tree

Answer 14

Use a recursive function to find the maximum depth of the left and right subtrees.

Question 15

Maximum Depth of Binary Tree,https://leetcode.com/problems/maximum-depth-of-binary-tree

Answer 15

Use a recursive function to find the maximum depth.

Question 16

Binary Tree Level Order Traversal,https://leetcode.com/problems/binary-tree-level-order-traversal

Answer 16

Use a queue to traverse the tree level by level.

Question 17

Lowest Common Ancestor of a Binary Tree,https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree

Answer 17

Use a recursive function to find the lowest common ancestor.

Question 18

Binary Tree Right Side View,https://leetcode.com/problems/binary-tree-right-side-view

Answer 18

Use a queue to traverse the tree level by level and add the rightmost node of each level to the result.

Question 19

Construct Binary Tree from Preorder and Inorder Traversal,https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal

Answer 19

Use a recursive function to build the tree.

Question 20

Serialize and Deserialize Binary Tree,https://leetcode.com/problems/serialize-and-deserialize-binary-tree

Answer 20

Use a queue to traverse the tree and serialize/deserialize it.

Question 21

Valid Palindrome,https://leetcode.com/problems/valid-palindrome

Answer 21

Use two pointers to check if the string is a palindrome.

Question 22

Valid Anagram,https://leetcode.com/problems/valid-anagram

Answer 22

Use a hash map to count the frequency of each character.

Question 23

Longest Palindrome,https://leetcode.com/problems/longest-palindromic-substring

Answer 23

Use dynamic programming to find the longest palindromic substring.

Question 24

Longest Substring Without Repeating Characters,https://leetcode.com/problems/longest-substring-without-repeating-characters

Answer 24

Use a sliding window to find the longest substring.

Question 25

String to Integer (atoi),https://leetcode.com/problems/string-to-integer-atoi

Answer 25

Handle whitespaces, signs, and overflow while converting the string to an integer.

Question 26

Longest Palindromic Substring,https://leetcode.com/problems/longest-palindromic-substring

Answer 26

Use dynamic programming to find the longest palindromic substring.

Question 27

Find All Anagrams in a String,https://leetcode.com/problems/find-all-anagrams-in-a-string

Answer 27

Use a sliding window and a hash map to find all anagrams.

Question 28

Minimum Window Substring,https://leetcode.com/problems/minimum-window-substring

Answer 28

Use a sliding window and a hash map to find the minimum window substring.

Question 29

Flood Fill,https://leetcode.com/problems/flood-fill

Answer 29

Use a queue or recursive function to perform the flood fill.

Question 30

01 Matrix,https://leetcode.com/problems/01-matrix

Answer 30

Use a queue to perform a breadth-first search from all zero cells.

Question 31

Clone Graph,https://leetcode.com/problems/clone-graph

Answer 31

Use a depth-first search to clone the graph.

Question 32

Course Schedule,https://leetcode.com/problems/course-schedule

Answer 32

Use a depth-first search to detect cycles in the graph.

Question 33

Number of Islands,https://leetcode.com/problems/number-of-islands

Answer 33

Use a depth-first search to count the number of islands.

Question 34

Rotting Oranges,https://leetcode.com/problems/rotting-oranges

Answer 34

Use a queue to perform a breadth-first search from all rotten oranges.

Question 35

Accounts Merge,https://leetcode.com/problems/accounts-merge

Answer 35

Use a union-find data structure to merge the accounts.

Question 36

Word Search,https://leetcode.com/problems/word-search

Answer 36

Use a depth-first search to find the word in the grid.

Question 37

Minimum Height Trees,https://leetcode.com/problems/minimum-height-trees

Answer 37

Use a breadth-first search to find the minimum height trees.

Question 38

Word Ladder,https://leetcode.com/problems/word-ladder

Answer 38

Use a breadth-first search to find the shortest transformation sequence.

Question 39

Valid Parentheses,https://leetcode.com/problems/valid-parentheses

Answer 39

Use a stack to check if the parentheses are valid.

Question 40

Implement Queue using Stacks,https://leetcode.com/problems/implement-queue-using-stacks

Answer 40

Use two stacks to implement a queue.

Question 41

Evaluate Reverse Polish Notation,https://leetcode.com/problems/evaluate-reverse-polish-notation

Answer 41

Use a stack to evaluate the reverse Polish notation.

Question 42

Min Stack,https://leetcode.com/problems/min-stack

Answer 42

Use two stacks to implement a min stack.

Question 43

Trapping Rain Water,https://leetcode.com/problems/trapping-rain-water

Answer 43

Use two pointers to find the amount of trapped rainwater.

Question 44

Basic Calculator,https://leetcode.com/problems/basic-calculator

Answer 44

Use a stack to evaluate the expression.

Question 45

Largest Rectangle in Histogram,https://leetcode.com/problems/largest-rectangle-in-histogram

Answer 45

Use a stack to find the largest rectangle.

Question 46

Merge Two Sorted Lists,https://leetcode.com/problems/merge-two-sorted-lists

Answer 46

Use two pointers to merge the lists.

Question 47

Linked List Cycle,https://leetcode.com/problems/linked-list-cycle

Answer 47

Use two pointers to detect a cycle in the linked list.

Question 48

Reverse Linked List,https://leetcode.com/problems/reverse-linked-list

Answer 48

Use a recursive or iterative function to reverse the linked list.

Question 49

Middle of the Linked List,https://leetcode.com/problems/middle-of-the-linked-list

Answer 49

Use two pointers to find the middle of the linked list.

Question 50

LRU Cache,https://leetcode.com/problems/lru-cache

Answer 50

Use a hash map and a doubly linked list to implement the LRU cache.

Question 51

Binary Search,https://leetcode.com/problems/binary-search

Answer 51

Use binary search to find the target element.

Question 52

First Bad Version,https://leetcode.com/problems/first-bad-version

Answer 52

Use binary search to find the first bad version.

Question 53

Search in Rotated Sorted Array,https://leetcode.com/problems/search-in-rotated-sorted-array

Answer 53

Use binary search to find the target element.

Question 54

Time Based Key-Value Store,https://leetcode.com/problems/time-based-key-value-store

Answer 54

Use a hash map and a list to store the key-value pairs.

Question 55

Maximum Profit in Job Scheduling,https://leetcode.com/problems/maximum-profit-in-job-scheduling

Answer 55

Use dynamic programming to find the maximum profit.

Question 56

Lowest Common Ancestor of a Binary Search Tree,https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree

Answer 56

Use a recursive function to find the lowest common ancestor.

Question 57

Validate Binary Search Tree,https://leetcode.com/problems/validate-binary-search-tree

Answer 57

Use a recursive function to validate the binary search tree.

Question 58

Kth Smallest Element in a BST,https://leetcode.com/problems/kth-smallest-element-in-a-bst

Answer 58

Use an inorder traversal to find the kth smallest element.

Question 59

Climbing Stairs,https://leetcode.com/problems/climbing-stairs

Answer 59

Use dynamic programming to find the number of ways to climb the stairs.

Question 60

Maximum Subarray,https://leetcode.com/problems/maximum-subarray

Answer 60

Use dynamic programming to find the maximum subarray.

Question 61

Coin Change,https://leetcode.com/problems/coin-change

Answer 61

Use dynamic programming to find the minimum number of coins.

Question 62

Partition Equal Subset Sum,https://leetcode.com/problems/partition-equal-subset-sum

Answer 62

Use dynamic programming to partition the array.

Question 63

Unique Paths,https://leetcode.com/problems/unique-paths

Answer 63

Use dynamic programming to find the number of unique paths.

Question 64

Ransom Note,https://leetcode.com/problems/ransom-note

Answer 64

Use a hash map to count the frequency of each character.

Question 65

Add Binary,https://leetcode.com/problems/add-binary

Answer 65

Use a loop to add the binary numbers.

Question 66

Spiral Matrix,https://leetcode.com/problems/spiral-matrix

Answer 66

Use four pointers to traverse the matrix in a spiral order.

Question 67

K Closest Points to Origin,https://leetcode.com/problems/k-closest-points-to-origin

Answer 67

Use a priority queue to find the k closest points.

Question 68

Task Scheduler,https://leetcode.com/problems/task-scheduler

Answer 68

Use a priority queue to schedule the tasks