Trapping Rain Water
Calculate water trapped between elevation bars Input: array of non-negative integers representing heights Output: total water units trapped
Trapping Rain Water is about turning repeated work into a reusable state instead of re-solving subproblems. It's a hard one for a reason: it stresses state-and-transition thinking when the rules pile up. The trap is picking a state that's too vague; keep it minimal and consistent.
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