Trapping Rain Water

Trapping Rain Water - LeetCodeLeetCode

Problem

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

Pseudocode

- dp
    - instead of finding each and every dip (local minima)
    - use subtractive approach

Solution

var trap = function (height) {
  let landArea = 0;
  let maxFromLeft = 0;
  let maxAreaFromLeft = 0;

  for (let h of height) {
    landArea += h;
    maxFromLeft = Math.max(maxFromLeft, h);
    maxAreaFromLeft += maxFromLeft;
  }

  let maxFromRight = 0;
  let maxAreaFromRight = 0;

  for (let i = height.length - 1; i >= 0; i--) {
    maxFromRight = Math.max(maxFromRight, height[i]);
    maxAreaFromRight += maxFromRight;
  }

  const boundingArea = height.length * maxFromLeft;
  const leftVoid = boundingArea - maxAreaFromLeft;
  const rightVoid = boundingArea - maxAreaFromRight;
  return boundingArea - leftVoid - rightVoid - landArea;
};

Time and Space Complexity

Time

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Space

• What did the code do

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