DSA CHEAT SHEET — Save This Post!
Most Asked Patterns in TCS Infosys Amazon Interviews!

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90% of coding interviews use THESE 10 patterns.
Master these = crack any tech interview!

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PATTERN 1: TWO POINTERS
Use when: Sorted array, find pairs, remove duplicates
Problems: Two Sum, Reverse String, 3Sum
Logic: left=0, right=n-1, move based on condition
Companies: Amazon, Microsoft, TCS

PATTERN 2: SLIDING WINDOW
Use when: Subarray/substring with condition
Problems: Max sum subarray, Longest substring
Logic: Expand right, shrink left when invalid
Companies: Infosys, Wipro, Google

PATTERN 3: BINARY SEARCH
Use when: Sorted array, find position/condition
Problems: Search in rotated array, Find peak
Logic: mid = (lo+hi)//2, eliminate half each time
Companies: Amazon, Flipkart, Accenture

PATTERN 4: LINKED LIST (Fast & Slow Pointer)
Use when: Cycle detection, find middle
Problems: Detect cycle, Find middle, Palindrome
Logic: slow moves 1 step, fast moves 2 steps
Companies: TCS, Infosys, HCL

PATTERN 5: TREE BFS (Level Order)
Use when: Level-by-level traversal, shortest path
Problems: Level order, Zigzag, Right side view
Logic: Use queue, process level by level
Companies: Amazon, Cognizant, Capgemini

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PATTERN 6: TREE DFS
Use when: Path sum, depth, validate BST
Problems: Max depth, Path sum, Inorder traversal
Logic: Recursion — visit node, left, right
Companies: Microsoft, Wipro, IBM

PATTERN 7: DYNAMIC PROGRAMMING
Use when: Optimization, count ways, max/min
Problems: Fibonacci, Knapsack, LCS, Coin change
Logic: Break into subproblems, store results
Companies: Amazon, Goldman Sachs, Barclays

PATTERN 8: STACK
Use when: Matching brackets, next greater element
Problems: Valid Parentheses, Stock Span, Min Stack
Logic: Push/pop based on LIFO order
Companies: TCS, Accenture, Infosys

PATTERN 9: HASHING (HashMap)
Use when: Count frequency, find duplicates, grouping
Problems: Two Sum, Anagram, Group Anagrams
Logic: key=element, value=count/index
Companies: Google, Amazon, Flipkart

PATTERN 10: GREEDY
Use when: Local optimal = global optimal
Problems: Activity selection, Jump game, Intervals
Logic: Always pick the best option at each step
Companies: Wipro, HCL, Mindtree

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MUST KNOW COMPLEXITY:

Array access -> O(1)
Binary Search -> O(log n)
Linear Search -> O(n)
Bubble Sort -> O(n2)
Merge/Quick Sort-> O(n log n)
HashMap get/put -> O(1) average
BFS/DFS -> O(V + E)

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30-DAY DSA PLAN FOR PLACEMENTS:

Week 1: Arrays + Strings + Hashing
Week 2: Linked List + Stack + Queue
Week 3: Trees + Binary Search
Week 4: DP + Greedy + Mock Tests

Practice on: LeetCode / GeeksForGeeks
Target: 2 problems daily = 60 problems/month

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