TrappingRainWater.js

/**
 * @author Anirudh Sharma
 * 
 * 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.
 * 
 * Constraints:
 * 
 * n == height.length
 * 0 <= n <= 3 * 10^4
 * 0 <= height[i] <= 10^5
 */
const trap = (height) => {
    // Special cases
    if (height === undefined || height.length < 2) {
        return 0;
    }
    // Total water that can be trapped
    let totalWater = 0;
    // Variables to store left max and right max
    let leftMax = 0;
    let rightMax = 0;
    // Left and right pointers
    let left = 0;
    let right = height.length - 1;
    // Loop until two pointers meet
    while (left <= right) {
        // If left wall is smaller than the right wall
        if (height[left] < height[right]) {
            if (height[left] > leftMax) {
                leftMax = height[left];
            } else {
                totalWater += leftMax - height[left];
            }
            left++;
        }
        // If right wall is smaller than the left wall
        else {
            if (height[right] > rightMax) {
                rightMax = height[right];
            } else {
                totalWater += rightMax - height[right];
            }
            right--;
        }
    }
    return totalWater;
};


console.log(trap([0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1]));
console.log(trap([4, 2, 0, 3, 2, 5]));