Leetcode for Beginners: 30 Leetcode Challenge

Learn to use a hash map or dictionary to optimize search from linear time to constant time, and choose maps or hash sets based on data uniqueness.

Learn to solve contains duplicates by scanning the array with a hash set, returning true when a seen element reappears and false if all values are unique, achieving O(n) time.

Determine if one string is an anagram of another using a character count map and a length check. Decrement counts as you scan, returning true when all counts balance.

Create a set from the jewels string and count stones that appear in it, honoring case sensitivity, to achieve O(M+N) time using hash set lookups.

Learn palindrome permutation in leetcode for beginners by using a set to track character pairs and determine if a string can form a palindrome, with at most one odd character.

Discover how to compute the length of the longest palindrome from a string of lowercase and uppercase letters by forming pairs and allowing a single center character.

Solve the group anagram homework problem by implementing a solution and documenting its time and space complexity, while understanding that anagrams group words by rearranging letters.

learn how to recognize a Toeplitz matrix, where every top-left to bottom-right diagonal has equal elements, and implement an efficient check by comparing each element with its diagonal neighbor.

Learn to transpose a 2d matrix by swapping row and column indices, building a new matrix, and iterating over all elements to produce the transpose.

Solve valid parentheses with a stack by pushing on left brackets and popping to match right brackets, ensuring correct nesting for (), [], and {} in linear time and space.

Learn to design a min stack that supports push, pop, top, and get minimum in constant time by using two stacks to track elements and current minima.

Solve the remove outermost parentheses problem using a stack, as part of Leetcode for Beginners. Share your solution and its time and space complexity in the comments for feedback.

Learn to implement a queue-based recent calls counter that tracks how many pings occur within a 3000‑millisecond window, updating with each new ping and discarding outdated ones.

Explain moving average from a data stream using a sliding window of fixed size, maintaining a queue and running sum to compute the last k values' average.

Implement a stack using a queue and analyze time and space complexity for push, pop, top, and empty operations, with guidance available via comments.

Learn how binary search finds a target in a sorted ascending array by comparing the middle element, updating low and high, and returning the index or -1 when absent.

Solve the search insert position problem on a sorted array of distinct integers by returning the index if found, or the insert position when not, using binary search.

Identify the peak index in a mountain array using binary search. Document time and space complexity in your Leetcode for Beginners homework solution.

Learn to find the middle of a linked list using the slow and fast pointer pattern, with a one-pass solution returning the middle or second middle node.

Remove the nth node from the end of a linked list using a two-pass approach or a one-pass slow-fast pointer method with a dummy node and re-link pointers.

Tackle the plus one in a linked list homework by analyzing time and space complexity, posting your solution with complexity in the comments, and seeking help if needed.

Compare two binary tree roots, P and Q, to determine if they are structurally identical with equal node values, using a recursive approach with null and value checks.

Implement binary tree inorder traversal using recursion, visiting left subtree, then node, then right subtree, collecting values in an output list with time complexity O(n).

determine if a binary tree is symmetric by recursively comparing left and right subtrees as mirrors, checking nulls and node values, and comparing left-left with right-right and left-right with right-left.

Solve a binary tree level-order traversal homework, traversing nodes level by level. Note time and space complexity in the comments, and post questions if you need help.

Learn how to insert a node into a binary search tree by finding the correct position, comparing values, and inserting at a null child using a recursive approach.

Search the root of a binary search tree for a target value and return the subtree rooted at that node; if not found, return null, using recursive left-right comparisons.

Validate a binary search tree by checking left subtree is < root and right subtree is > root; return a boolean and time and space complexity in comment box.

Learn to solve the last stone weight problem using a max heap to smash the two heaviest stones and insert their difference, returning the final weight or zero.

Use a min heap of size k to find the kth largest element: loop the array, add to a priority queue, remove the smallest when over k, and return top.

solve the k closest points to origin homework problem using a heap, and analyze the euclidean distance and time and space complexity for beginners.