After grinding hundreds of LeetCode problems and clearing technical interviews, I have distilled the most effective patterns for DSA preparation. This guide focuses on what actually appears in real interviews at top tech companies.
The Core Patterns You Must Know
Most DSA interview problems reduce to one of ~15 patterns. Master these and you can solve most problems you have never seen before:
- Sliding Window — for substring/subarray problems with a contiguous constraint
- Two Pointers — for sorted arrays and linked list problems
- Fast & Slow Pointers — cycle detection in linked lists and arrays
- Tree BFS/DFS — level-order traversal, path sum, subtree problems
- Dynamic Programming — overlapping subproblems (Fibonacci, knapsack, LCS)
Array Mastery: The Foundation
Arrays appear in 60%+ of interview questions. Focus on prefix sums, frequency maps (HashMap), and in-place operations to avoid O(n) extra space.
// Prefix sum for range queries in O(1)
int[] prefix = new int[n + 1];
for (int i = 0; i < n; i++) prefix[i+1] = prefix[i] + arr[i];
int rangeSum = prefix[r+1] - prefix[l];Graph Problems: BFS vs DFS
Use BFS for shortest path problems, DFS for connectivity, cycle detection, and topological sort. Always track visited nodes to avoid infinite loops.
Time & Space Complexity Mindset
Every solution you write should come with a clear explanation of its time and space complexity. Interviewers value reasoning over brute-force correct code. A clean O(n log n) solution explained well beats messy O(n) code every time.
"Master the pattern, not the problem." — Every successful SDE interviewer
Recommended Study Order
Week 1–2: Arrays, Strings, HashMaps. Week 3–4: Two Pointers, Sliding Window, Recursion. Week 5–6: Trees, Graphs, BFS/DFS. Week 7–8: Dynamic Programming, Backtracking. Week 9–10: System Design basics and mock interviews.