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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

object Solution {
    def trap(height: Array[Int]): Int = {
        var left = 0
        var right = height.length - 1
        var result = 0
        var lowerWall = 0

        while (left < right) {
            val leftValue = height(left)
            val rightValue = height(right)

            if (leftValue < rightValue) {
                lowerWall = Math.max(leftValue, lowerWall)
                result += lowerWall - leftValue
                left += 1
            } else {
                lowerWall = Math.max(rightValue, lowerWall)
                result += lowerWall - rightValue
                right -= 1
            }
        }

        result
    }
}

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