State
dp[i][j] = # paths to reach (i,j).
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State
dp[i][j] = # paths to reach (i,j).
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Transition
dp[i][j] = dp[i-1][j] + dp[i][j-1]. Came from up OR left.
Base cases
First row + first column all = 1 (only one way).
Closed form
C(M+N-2, M-1) — pure combinatorics. But DP handles obstacles case.