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Trapping Rain Water

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining.

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Pythontime O(n) · space O(1)

def trap(height: list[int]) -> int:
    l, r = 0, len(height) - 1
    maxL, maxR = height[l], height[r]
    water = 0
    while l < r:
        if maxL <= maxR:
            l += 1
            maxL = max(maxL, height[l])
            water += maxL - height[l]
        else:
            r -= 1
            maxR = max(maxR, height[r])
            water += maxR - height[r]
    return water

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