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;
};
