Swim in Rising Water
Given n x n grid where grid[i][j] is elevation at (i,j) At time t, can swim if elevation <= t Find minimum time to reach bottom-right from top-left Example: grid = [[0,2],[1,3]] => 3
Swim in Rising Water is really a boundary-finding problem, even if it doesn't look like it at first. It's a hard one for a reason: it stresses boundary precision when the rules pile up. If you can state the invariant for your search space, you'll stop making off-by-one mistakes.
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