dsa1 min read
Word Break — Reachability DP
Check if string can be segmented into dictionary words. DP: dp[i]=true if some dp[j] is true and s[j:i] is in wordDict.
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Longest Increasing Subsequence — O(n log n) Patience Sorting
Find the length of the longest strictly increasing subsequence. O(n²) DP or O(n log n) patience sorting with binary search.
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Russian Doll Envelopes — 2D LIS
Maximum envelopes you can nest. Sort by width ascending and height descending, then LIS on heights. Descending heights prevents using same-width twice.
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Palindromic Substrings — Expand Around Center
Count palindromic substrings. Expand around each center (n odd-length + n-1 even-length centers), count valid expansions.
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Longest Palindromic Subsequence — 2D DP
Find longest palindromic subsequence length. 2D DP: if s[i]==s[j], dp[i][j] = dp[i+1][j-1]+2, else max of neighbors. Fill diagonally.
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