LeetCode-in-All

42. Trapping Rain Water

Hard

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 above 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 * 104
• 0 <= height[i] <= 105

Solution

class Solution {
    fun trap(height: IntArray): Int {
        val size = height.size
        var maxLeft = 0
        var maxRight = 0
        var totalWater = 0
        var leftPtr = 0
        var rightPtr = size - 1
        while (leftPtr < rightPtr) {
            if (height[leftPtr] <= height[rightPtr]) {
                if (maxLeft > height[leftPtr]) {
                    totalWater = totalWater + maxLeft - height[leftPtr]
                } else {
                    maxLeft = height[leftPtr]
                }
                ++leftPtr
            } else {
                if (maxRight > height[rightPtr]) {
                    totalWater = totalWater + maxRight - height[rightPtr]
                } else {
                    maxRight = height[rightPtr]
                }
                --rightPtr
            }
        }
        return totalWater
    }
}

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