Google Coding Interview Questions
26 Google coding interview problems with full optimal solutions — 2 easy, 13 medium, 11 hard. Every problem ships with multiple approaches (brute-force first, then the optimal), complexity tables for each, company-specific tips on what an Google interviewer values, and a FAQ section.
- #1easyfoundational
1. Two Sum
Two Sum is Google's canonical warm-up: given an integer array and a target, return the indices of the two numbers that add up to the target. The interviewer is grading your willingness to narrate the space-for-time tradeoff before jumping to the optimal.
4 free resourcesSolve → - #3mediumfrequently asked
3. Longest Substring Without Repeating Characters
Given a string, return the length of the longest substring with all unique characters. Google asks this to test sliding-window intuition and whether you can articulate why O(n) is achievable when the naive answer is O(n^3).
- #4hardless common
4. Median of Two Sorted Arrays
Given two sorted arrays, return the median of the combined array in O(log(min(m, n))). Google asks this to test whether you can apply binary search on the smaller array's partition position — one of the hardest applications of the binary-search paradigm.
- #5mediumfrequently asked
5. Longest Palindromic Substring
Find the longest palindromic substring in a given string. Google asks this to test whether you reach for the expand-around-center technique and can articulate why it's O(n^2) instead of the naive O(n^3).
- #10hardless common
10. Regular Expression Matching
Implement regex matching with '.' and '*'. Google asks this to test whether you can set up a 2D DP for a problem where the recurrence has multiple branches depending on what character pattern[j-1] is.
- #20easyfoundational
20. Valid Parentheses
Given a string of brackets, return true if every opener has a matching, correctly-nested closer. Google asks this to confirm you can model push/pop state with a stack and articulate why the LIFO property is what makes the problem cleanly solvable.
- #23hardfrequently asked
23. Merge k Sorted Lists
Given k sorted linked lists, merge them into one sorted linked list. Google asks this to see whether you reach for a min-heap, can articulate the n log k vs nk tradeoff, and know how to compare two linked-list nodes inside a priority queue.
- #33mediumfrequently asked
33. Search in Rotated Sorted Array
Search for a target in a sorted array that has been rotated at some unknown pivot. Google asks this to test whether you can apply modified binary search and reason about which half is sorted at each step.
- #42hardfrequently asked
42. Trapping Rain Water
Given heights, compute how much water trapped between bars after rain. Google asks this to see whether you reach for the two-pointer invariant (water at i depends only on min(maxLeft, maxRight)) without falling into the O(n^2) trap of recomputing maxes per index.
- #53mediumfrequently asked
53. Maximum Subarray
Find the contiguous subarray with the largest sum. Google asks this to see whether you arrive at Kadane's algorithm and can articulate the invariant: at index i, the best subarray ending here is either the current element alone or the current element extending the previous best.
- #68hardfrequently asked
68. Text Justification
Given an array of words and a max line width, format the text so each line is fully justified. Google asks this to see whether you can manage the edge cases (last line, single-word lines, uneven space distribution) without losing track.
- #72hardfrequently asked
72. Edit Distance
Given two strings, return the minimum number of insertions, deletions, and substitutions to transform one into the other. Google asks this to test whether you can set up a 2D DP recurrence cleanly and articulate why each character pair has exactly four sub-choices.
- #91mediumfrequently asked
91. Decode Ways
Given a digit string, count the ways it can be decoded as letters (A=1...Z=26). Google asks this to test whether you reach for 1D DP and can handle the edge cases around '0' digits which can't stand alone.
- #124hardfrequently asked
124. Binary Tree Maximum Path Sum
Find the maximum sum path in a binary tree, where the path can start and end at any node. Google asks this to test whether you can separate two concepts at the same recursive node: the best path that 'extends upward' versus the best path that 'turns at this node and goes nowhere else.'
- #127hardfrequently asked
127. Word Ladder
Given a begin word, end word, and word list, return the length of the shortest transformation sequence where each step changes one letter. Google asks this to test whether you reach for BFS on an implicit graph and can articulate why BFS (not DFS) is the right tool for shortest-path on unweighted edges.
- #128mediumfrequently asked
128. Longest Consecutive Sequence
Given an unsorted array, find the length of the longest consecutive elements sequence. The catch: Google requires O(n) — sorting (O(n log n)) is the wrong answer and you must reach for the hash-set 'start a chain only from the smallest element' trick.
- #139mediumfrequently asked
139. Word Break
Given a string and a dictionary, return true if the string can be segmented into dictionary words. Google asks this to test whether you reach for DP and can articulate why a hash set of words + a boolean DP table gives O(n^2) instead of the exponential backtracking blowup.
- #146mediumfrequently asked
146. LRU Cache
Design a cache that evicts the least recently used key when full. Google asks this to test whether you reach for the doubly-linked-list + hash map combo and can articulate why neither data structure alone gives you O(1) for both get and put.
- #207mediumfrequently asked
207. Course Schedule
Given courses and prerequisites, return true if all courses can be finished. Google asks this to test whether you reach for cycle detection in a directed graph and can articulate the difference between DFS-with-coloring and Kahn's BFS topological sort.
- #212hardfrequently asked
212. Word Search II
Given a 2D board and a list of words, return all words on the board. Google asks this to test whether you reach for a trie + DFS combo and can articulate why it's drastically faster than running Word Search I once per word.
- #253mediumfrequently asked
253. Meeting Rooms II
Given meeting intervals, return the minimum number of rooms required. Google asks this to test whether you reach for a min-heap of end times or the sweep-line / two-pointer counter, and can articulate the start/end event framing.
- #295hardfrequently asked
295. Find Median from Data Stream
Maintain the median of a stream of numbers with O(log n) addNum and O(1) findMedian. Google asks this to test whether you reach for two heaps (max-heap of lower half + min-heap of upper half) and can articulate the size-balance invariant.
- #297hardfrequently asked
297. Serialize and Deserialize Binary Tree
Encode a binary tree to a string and decode it back. Google asks this to test whether you handle null markers correctly and choose a traversal order that both serialize and deserialize can agree on without ambiguity.
- #300mediumfrequently asked
300. Longest Increasing Subsequence
Return the length of the longest strictly increasing subsequence. Google asks this to test whether you reach for O(n log n) via patience sorting / binary search rather than stopping at the textbook O(n^2) DP.
- #380mediumfrequently asked
380. Insert Delete GetRandom O(1)
Design a data structure that supports insert, remove, and getRandom — all in O(1) average. Google asks this to test whether you can combine a hash map (for O(1) lookup) with a dynamic array (for O(1) random access), and to see if you handle the 'swap to end before pop' trick for O(1) removal.
- #399mediumfrequently asked
399. Evaluate Division
Given variable equations like a/b = 2.0 and b/c = 3.0, answer queries like a/c. Google asks this to test whether you can model the equations as a weighted graph and reach for DFS or union-find.
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