dsa2 min read
Merge Intervals — Sort + Linear Scan O(n log n) [Google, Meta, Amazon]
Merge all overlapping intervals. Sort by start, then merge greedily in one pass. O(n log n) time.
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dsa2 min read
Spiral Matrix — Direction Array Simulation [Google, Meta, Amazon]
Return all elements of an m×n matrix in spiral order. Use boundary shrinking or direction array. O(m*n) time O(1) space.
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dsa1 min read
Group Anagrams — Sort Key HashMap O(n*k log k) [Google, Meta, Amazon]
Group strings that are anagrams of each other. Use sorted string as HashMap key. O(n*k*log k) time. Full solutions in 5 languages.
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dsa2 min read
Longest Substring Without Repeating Characters — Sliding Window [Google, Amazon, Meta]
Find length of longest substring without repeating characters. Optimal O(n) sliding window with HashMap tracking last seen positions.
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dsa2 min read
Subarray Sum Equals K — Prefix Sum + HashMap O(n) [Google, Meta]
Count subarrays summing to k. Classic prefix sum + HashMap trick: if prefix[j]-prefix[i]=k then prefix[i]=prefix[j]-k. O(n) time.
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