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Unique Paths — Grid Path Count DP
Count paths from top-left to bottom-right moving only right or down. dp[i][j] = dp[i-1][j] + dp[i][j-1]. Math formula O(1) also works.
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Unique Paths II — Grid with Obstacles
Count paths avoiding obstacle cells. Same DP but obstacles block cell (dp[i][j]=0) and obstacle at start/end returns 0.
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Minimum Path Sum — Grid DP
Find minimum cost path from top-left to bottom-right moving right or down. dp[i][j] = grid[i][j] + min(dp[i-1][j], dp[i][j-1]).
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Triangle — Bottom-Up 1D DP
Minimum path sum from top to bottom of triangle. Bottom-up DP: dp[j] = triangle[i][j] + min(dp[j], dp[j+1]).
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Longest Common Subsequence — LCS 2D DP
Classic LCS: dp[i][j] = LCS length of s1[:i] and s2[:j]. If chars match add 1 to diagonal; else take max of skip either string.
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