42. Trapping Rain Water

Description

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

 

Solutions

Solution: Two Pointers

• Time complexity: O(n)
• Space complexity: O(1)

 

JavaScript

/**
 * @param {number[]} height
 * @return {number}
 */
const trap = function (height) {
  let left = (result = maxHeight = 0);
  let right = height.length - 1;

  while (left < right) {
    const minHeight = Math.min(height[left], height[right]);

    maxHeight = Math.max(minHeight, maxHeight);
    result += maxHeight - minHeight;
    minHeight === height[left] ? (left += 1) : (right -= 1);
  }
  return result;
};