42. Trapping Rain Water

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 {

    /**
     * @param Integer[] $height
     * @return Integer
     */
    function trap($height) {
        $result = 0;
        $leftIndex = 0;
        $rightIndex = count($height) - 1;
        $maxLeft = 0;
        $maxRight = 0;
        while ($leftIndex < $rightIndex) {
            if ($height[$leftIndex] < $height[$rightIndex]) {
                $maxLeft = max($maxLeft, $height[$leftIndex]);
                $result += $maxLeft - $height[$leftIndex];
                $leftIndex++;
            } else {
                $maxRight = max($maxRight, $height[$rightIndex]);
                $result += $maxRight - $height[$rightIndex];
                $rightIndex--;
            }
        }
        return $result;
    }
}

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