Trapping Rain Water

Hard

Two Pointers

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.

Example 1

Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]

Output: 6

Explanation: The elevation map (black section) is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.

Example 2

Input: height = [4,2,0,3,2,5]

Output: 9

Constraints

•n == height.length
•1 <= n <= 2 * 10^4
•0 <= height[i] <= 10^5

Show Hints (3)

Hint 1: For each position, the water trapped depends on the minimum of (max height to the left, max height to the right) minus the current height.

Hint 2: Use two pointers starting from both ends, tracking the maximum heights seen from each side.

Hint 3: Process from the side with the smaller maximum height, since that determines the water level at that position.

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