LeetCode 42. Trapping Rain Water (javascript)

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
• 0 <= n <= 3 * 104
• 0 <= height[i] <= 105

Idea:

Two pointers traversal

Solution:

/**
 * @param {number[]} height
 * @return {number}
 */
var trap = function(height) {
    let res = 0;
    const n = height.length;
    let l = 0;
    let r = n - 1;
    
    while (l < r) {
        let min = Math.min(height[l], height[r]);
        
        // if left side is smaller
        if (min === height[l]) {
            l++;
            while (l < r && height[l] < min) {
                res += min - height[l++];
            }
        } else { // right side is smaller
            r--;
            while (l < r && height[r] < min) {
                res += min - height[r--];
            }
        }
    }
    return res;
};