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Quick-Fire 50 - Interview Cheatsheet
The 50 problems that show up most in FAANG + Indian tech interviews (Amazon, Google, Meta, PhonePe, Flipkart, Uber). Bookmark this page and review before every interview.
Which list should I use?
1 week of prep? → You’re here. Quick-Fire 50.4 weeks of prep? → The Blind 75 (includes all 50 + 25 more)8 weeks of prep? → NeetCode 150 (most comprehensive)Already confident on the patterns? → The Hard List (hard-only, all 19 patterns)
💡 How to use: Scan the “Approach” column. If you can’t explain it in 10 seconds, that’s the one to practice.
Arrays & Hashing
Two Pointers
#
Problem
Approach
Time
Space
9
Valid Palindrome Left/right pointers, skip non-alphanumeric, compare
O(n)
O(1)
10
3Sum Sort + fix one, two-pointer on rest. Skip duplicates.
O(n²)
O(1)
11
Container With Most Water Left/right, move the shorter side inward
O(n)
O(1)
12
Trapping Rain Water Two pointers with leftMax/rightMax tracking
O(n)
O(1)
Sliding Window
Stack
Binary Search
Linked List
Trees
Graphs
#
Problem
Approach
Time
Space
35
Number of Islands BFS/DFS from each ‘1’, mark visited
O(m×n)
O(m×n)
36
Clone Graph BFS/DFS + HashMap (old node → new node)
O(V+E)
O(V)
37
Course Schedule Topological sort (BFS Kahn’s or DFS cycle detection)
O(V+E)
O(V+E)
38
Word Ladder BFS level-by-level, change one char at a time
O(n·26·L)
O(n·L)
39
Dijkstra’s Shortest Path Min-heap + relaxation
O((V+E) log V)
O(V)
Dynamic Programming
#
Problem
Approach
Time
Space
40
Climbing Stairs dp[i] = dp[i-1] + dp[i-2] (Fibonacci)O(n)
O(1)
41
Coin Change dp[i] = min(dp[i-coin]+1) for each coinO(amount×coins)
O(amount)
42
Longest Increasing Subsequence DP: O(n²) or patience sort with binary search: O(n log n)
O(n log n)
O(n)
43
Word Break dp[i] = any dp[j] && s[j:i] in dictO(n²·L)
O(n)
44
House Robber dp[i] = max(dp[i-1], dp[i-2] + nums[i])O(n)
O(1)
45
Edit Distance 2D DP: insert/delete/replace at each (i,j)
O(m×n)
O(m×n)
Greedy / Intervals
#
Problem
Approach
Time
Space
46
Merge Intervals Sort by start, extend end if overlapping
O(n log n)
O(n)
47
Meeting Rooms II Sort starts + ends separately, sweep with counter
O(n log n)
O(n)
48
Jump Game Track farthest reachable index
O(n)
O(1)
49
Non-Overlapping Intervals Sort by end, greedily keep earliest-ending
O(n log n)
O(1)
50
Task Scheduler Count max-freq task, compute idle slots
O(n)
O(26)
Pattern Recognition Cheatsheet
If you see…
Try this pattern
“Sorted array + find pair/target”
Two Pointers
“Contiguous subarray/substring with constraint”
Sliding Window
“Find minimum/maximum that satisfies condition”
Binary Search on Answer
“Shortest path in unweighted graph”
BFS
“All combinations/permutations”
Backtracking (DFS)
“Overlapping subproblems + optimal substructure”
Dynamic Programming
“Next greater/smaller element”
Monotonic Stack
“Connected components / merge groups”
Union-Find
“Task dependencies / ordering”
Topological Sort
“Prefix matching / dictionary”
Trie
“Intervals overlap/merge”
Sort by start + sweep
“Top K / Kth largest”
Heap (Priority Queue)
Complexity Quick Reference
Structure
Access
Search
Insert
Delete
Array
O(1)
O(n)
O(n)
O(n)
HashMap
-
O(1) avg
O(1) avg
O(1) avg
BST (balanced)
-
O(log n)
O(log n)
O(log n)
Heap
O(1) peek
O(n)
O(log n)
O(log n)
Stack/Queue
O(1) top
O(n)
O(1)
O(1)
Sort
Best
Average
Worst
Stable?
Merge Sort
O(n log n)
O(n log n)
O(n log n)
Yes
Quick Sort
O(n log n)
O(n log n)
O(n²)
No
Heap Sort
O(n log n)
O(n log n)
O(n log n)
No
Tim Sort (Java/Python default)
O(n)
O(n log n)
O(n log n)
Yes
The 10 “Must-Solve-First” Problems
If you only have 3 days, solve these - they cover the most patterns with the least problems:
Two Sum (HashMap)Best Time to Buy/Sell Stock (Greedy/Kadane)Longest Substring Without Repeating (Sliding Window)3Sum (Two Pointers + Sort)Merge Intervals (Sort + Sweep)Number of Islands (BFS/DFS)Coin Change (DP)LRU Cache (HashMap + DLL)Course Schedule (Topological Sort)Valid Parentheses (Stack)
Bookmark this page. Review it 30 minutes before every coding interview.
Want detailed solutions? Check the DSA problems section .