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Trapping Rain Water is a hard coding interview question covering arrays, two pointers — the kind asked at FAANG (Facebook/Meta, Apple, Amazon, Netflix, Google) and AI-first companies like Anthropic, OpenAI, and Nvidia.
Given n non-negative integers representing an elevation map where each bar has width 1, compute how much water it can trap after raining.
Example: [0,1,0,2,1,0,1,3,2,1,2,1] -> 6
Solve in O(n) time and O(1) space. Explain why the two-pointer approach works.
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