The classic warm-up: find two indices that add to a target — brute force, hash map, and the follow-ups interviewers love.

beginner · Google · Amazon · Meta

Detect any duplicate in an array — Set vs sort, O(n) hash, and follow-ups like nearby duplicates (k-window).

beginner · Google · Meta

Same characters with same counts — frequency array or sort both strings, O(n) or O(n log n).

beginner · Google · Meta · Amazon

Bucket strings by sorted signature or char counts — hash map of anagram groups, complexity, and interview variants.

intermediate · Google · Meta · Amazon

k most frequent numbers — count with a map, then bucket sort by frequency for average O(n).

intermediate · Google · Meta · Amazon

Output[i] = product of all nums except i — prefix/suffix passes, O(n) time without division.

intermediate · Google · Meta · Amazon

Longest run of consecutive integers in unsorted array — Set + only start at run heads for O(n) average.

intermediate · Google · Meta · Amazon

Alphanumeric palindrome check — two pointers skipping non-alphanumerics, O(n) time O(1) space.

beginner · Google · Meta · Amazon

Find all unique triplets that sum to zero — sort + two pointers, deduping strategy, and complexity tradeoffs.

intermediate · Google · Meta · Amazon

Two pointers on heights for max water area — why you move the shorter side, brute O(n²) vs O(n), and edge cases.

intermediate · Google · Meta · Amazon

One buy, one sell — track min price so far and max profit in a single O(n) pass. The classic stock DP warm-up.

beginner · Google · Meta · Amazon

Find the longest substring with all unique characters — sliding window with a last-seen map, O(n) time.

intermediate · Google · Meta · Amazon

Longest substring replaceable into one character with ≤ k swaps — sliding window, max-frequency tracking, O(n).

intermediate · Google · Meta

Smallest window in s covering all of t — sliding window with need/have counts, O(|s| + |t|).

advanced · Google · Meta

Stack with O(1) push, pop, top, and getMin — pair each value with the min so far, or dual stacks.

intermediate · Google · Meta

Evaluate RPN with a stack — operands push, operators pop-two apply, integer division toward zero in JS.

intermediate · Google · Meta · Amazon

Check if brackets are valid with a stack — matching pairs, edge cases, and the interview follow-ups.

beginner · Google · Amazon · Meta

Days until a warmer temperature — monotonic decreasing stack of indices, O(n) next-greater-element pattern.

intermediate · Google · Meta

Cars race to a target — sort by position, compare time-to-target on a stack, count fleets that never catch each other.

intermediate · Google · Meta

Search a sorted array in O(log n) — classic template, boundary bugs, and the first-true / last-true variants.

beginner · Google · Meta · Amazon

Find target in a rotated sorted array of distinct ints — modified binary search on the sorted half.

intermediate · Google · Meta

Rotated sorted array min via binary search — compare mid to hi, no-duplicates case, and the with-duplicates follow-up.

intermediate · Google · Meta

set(key,value,timestamp) and get previous value — map of sorted timestamps, binary search on get.

intermediate · Google · Meta · Amazon

Reverse a singly linked list in place — iterative three-pointer walk, recursive version, and the interview follow-ups.

beginner · Google · Meta · Amazon

Merge two sorted linked lists into one sorted list — dummy head, two pointers, O(n+m) time.

beginner · Google · Meta · Amazon

Detect a cycle in a linked list — Set of nodes vs Floyd tortoise and hare O(1) space, plus find-entrance follow-up.

beginner · Google · Meta · Amazon

L0→Ln→L1→Ln-1… — find mid, reverse second half, merge alternating, O(n) time O(1) space.

intermediate · Google · Meta

Delete the nth node from the end of a list in one pass — two pointers with n-gap, dummy head.

intermediate · Google · Meta

Deep-copy a linked list with next and random pointers — hash map O(n) space vs interleaving nodes O(1) extra space.

intermediate · Google · Meta · Amazon

Add two numbers stored as reverse linked lists — digit-by-digit with carry, unequal lengths, and the final carry edge case.

intermediate · Google · Meta

Array of n+1 ints in 1..n with one duplicate — Floyd cycle detection O(1) space, binary search on counts alternative.

intermediate · Google · Meta

Design an LRU cache with O(1) get and put — Map + doubly linked list, Map insertion-order approach, and interview tradeoffs.

advanced · Google · Meta · Amazon

Prefix tree with insert, search, startsWith — node children map, end flag, and complexity for autocomplete-style use.

intermediate · Google · Meta · Amazon

Word dictionary with '.' wildcards — trie insert, recursive search over branches, and complexity tradeoffs.

intermediate · Google · Meta

Find all dictionary words on a board — Trie of words + DFS from each cell, prune dead branches.

advanced · Google · Meta · Amazon

Count islands in a grid with DFS or BFS flood fill — graph framing, complexity, and common grid bugs.

intermediate · Google · Meta · Amazon

Deep-copy an undirected connected graph — BFS/DFS with a map from original node to clone, neighbor wiring pitfalls.

intermediate · Google · Meta · Amazon

Largest 4-connected land component in a grid — DFS/BFS flood fill that returns area, not just count.

intermediate · Google · Meta

Cells that can reach both oceans — multi-source DFS/BFS inland from Pacific and Atlantic borders.

advanced · Google · Meta

Can you finish all courses given prerequisites — cycle detection in a directed graph via DFS colors or Kahn’s BFS.

intermediate · Google · Meta

Return a valid course order (topological sort) or empty array if a cycle exists — Kahn BFS and DFS postorder.

intermediate · Google · Meta

Find the edge that creates a cycle in a near-tree graph — Union-Find returns the last redundant edge.

intermediate · Google · Meta · Amazon

Shortest transformation beginWord→endWord changing one letter — BFS on implicit word graph.

advanced · Google · Meta · Amazon

n steps, take 1 or 2 at a time — Fibonacci DP, bottom-up O(1) space, and the recursion trap interviewers watch for.

beginner · Google · Meta

Max money from houses in a line without adjacent robs — DP recurrence, O(1) space roll, and circular follow-up.

intermediate · Google · Meta

Circular houses — max of rob linear range [0..n-2] vs [1..n-1], reusing House Robber I as a helper.

intermediate · Google · Meta

Longest palindromic substring — expand around centers O(n²), DP table alternative, and Manacher mention for O(n).

intermediate · Google · Meta · Amazon

Fewest coins to make amount — unbounded knapsack DP, BFS alternative, and why greedy fails on arbitrary denominations.

intermediate · Google · Meta · Amazon

Can s be segmented into dictionary words? DP boolean array, O(n² · word checks).

intermediate · Google · Meta · Amazon

LIS length — classic O(n²) DP and O(n log n) patience sorting with binary search tails array.

intermediate · Google · Meta

Robot on m×n grid moves only right/down — DP grid or combinatorial C(m+n-2, m-1).

intermediate · Google · Meta

Can you reach the last index — greedy farthest reach, DP alternative, and when zero cells trap you.

intermediate · Google · Meta

Minimum jumps to last index — BFS layers on the array, greedy end/far windows, O(n) without real queue.

intermediate · Google · Meta

Circular gas stations — if total gas ≥ cost a unique start exists; one-pass track tank and reset start on negative.

intermediate · Google · Meta · Amazon

Split hand into groups of W consecutive cards — sorted map/counting greedy, fail when a needed card is missing.

intermediate · Google · Meta · Amazon

Merge overlapping intervals — sort by start, linear scan, and the calendar/UI variants interviewers stack on top.

intermediate · Google · Meta · Amazon

Insert a new interval into sorted non-overlapping intervals — scan left, merge overlap, append right. O(n).

intermediate · Google · Meta

Minimum removals so no intervals overlap — greedy by end time, keep the interval that finishes first.

intermediate · Google · Meta · Amazon

Can one person attend all meetings? Sort by start and check adjacent overlaps — classic interval warm-up.

beginner · Google · Meta · Amazon

Minimum rooms for all meetings — sort starts and ends, sweep line counting concurrent intervals.

intermediate · Google · Meta · Amazon

Reverse the 32 bits of an integer — shift-and-accumulate or divide-and-conquer bit swaps.

beginner · Google · Meta

Hamming weight — count set bits with n & (n-1) Brian Kernighan, or shift-and-mask, O(set bits).

beginner · Google · Meta · Amazon

Count 1-bits for every number in [0, n] — Brian Kernighan per number, and O(n) DP using i >> 1 and i & 1.

intermediate · Google · Meta · Amazon

Find the missing number in 0..n — XOR all indices and values, or use Gauss sum, O(n) time O(1) space.

beginner · Google · Meta · Amazon

Add without + or − operators — XOR for sum bits, AND+shift for carry, loop until carry clears.

intermediate · Google · Meta · Amazon

Mirror a binary tree — recursive swap, BFS/DFS iterative, and why this is a five-minute interview handshake.

beginner · Google · Meta

Height of a binary tree via DFS recursion or BFS levels — the tree warm-up every interviewer trusts.

beginner · Google · Meta · Amazon

Check if two binary trees are identical in structure and values — recursive DFS or BFS pair walk.

beginner · Google · Meta

Is subRoot identical to some subtree of root? DFS every node and same-tree check, or serialize+find.

beginner · Google · Meta · Amazon

Find LCA of two nodes in a BST — walk from the root using ordering, O(h) time, iterative or recursive.

intermediate · Google · Meta · Amazon

BFS level-order traversal of a binary tree — queue sizing, empty levels, and the recursive DFS variant interviewers compare.

intermediate · Google · Meta · Amazon

Is the tree a valid BST? Carry (min, max) bounds down DFS, or inorder strictly increasing.

intermediate · Google · Meta

Kth smallest BST value via inorder — recursive count, iterative stack, and O(h) follow-up with node subtree sizes.

intermediate · Google · Meta · Amazon

Rebuild a binary tree from preorder and inorder arrays — root split, index map, and O(n) recursive construction.

advanced · Google · Meta

Hard tree DP: max path sum anywhere in a binary tree — gain from one child, global answer, and negative-node traps.

advanced · Google · Meta

Encode a binary tree to a string and rebuild it — preorder with null markers or BFS level order.

advanced · Google · Meta

FIFO queue from two LIFO stacks — amortized O(1) pop/peek by flushing input stack to output stack on demand.

beginner · Google · Meta

LIFO stack from FIFO queues — push O(n) rotate or pop O(n); single-queue rotation technique.

beginner · Google · Meta

Max of every window of size k — monotonic decreasing deque of indices, O(n) total.

advanced · Google · Meta · Amazon

Running median with two heaps — max-heap lows + min-heap highs, balance rules, and JS heap sketch.

advanced · Google · Meta · Amazon

K nearest points to (0,0) — sort O(n log n), max-heap of size k, and quickselect average O(n).

intermediate · Google · Meta · Amazon

Min time to run tasks with cooldown n — formula from max frequency, or simulate with a max-heap.

intermediate · Google · Meta · Amazon

postTweet, follow/unfollow, getNewsFeed — hash maps of tweets and follows, merge k recent feeds for top 10.

advanced · Google · Meta · Amazon

Serialize a list of strings to one string and back — length-prefix encoding that survives empty strings and delimiters.

intermediate · Google · Meta · Amazon

Validate a partial 9×9 board — track seen digits per row, column, and 3×3 box with sets.

intermediate · Google · Meta

Traverse an m×n matrix in spiral order — shrink four boundaries, O(mn) time.

intermediate · Google · Meta · Amazon

Rotate an n×n matrix 90° clockwise in place — transpose then reverse each row (or layer peel).

intermediate · Google · Meta

If a cell is 0, zero its row and column — mark in-place with first row/col flags, O(1) extra space.

intermediate · Google · Meta · Amazon

Does a word exist on a letter grid? Backtracking DFS from each cell with visited marks.

intermediate · Google · Meta · Amazon

Power set of distinct integers — backtracking include/exclude or bit masks, 2^n subsets.

intermediate · Google · Meta · Amazon

Backtracking to all unique combinations that sum to target — reuse allowed, sort + prune, and Combination Sum II contrast.

intermediate · Google · Meta

Generate all permutations of a distinct array — swap-based or used-mask backtracking, n! outputs.

intermediate · Google · Meta

Place n queens so none attack — backtracking with column and diagonal masks, return all board layouts.

advanced · Google · Meta · Amazon

Partition a string so every substring is a palindrome — backtrack with expand checks, list all partitions.

intermediate · Google · Meta · Amazon

Map digits 2-9 to letters and backtrack all strings — BFS build alternative, empty input, and pruning none needed.

intermediate · Google · Meta · Amazon