Trapping Rain Water

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

Solution

class Solution {

public int trap(int[] height) {

int len = height.length;

if(len == 0) {

return 0;

}

int[] leftMax = new int[len];

int[] rightMax = new int[len];

leftMax[0] = height[0];

for(int i=1; i<len; i++) {

leftMax[i] = Math.max(height[i], leftMax[i-1]);

}

rightMax[len-1] = height[len-1];

for(int i=len-2; i>=0; i--) {

rightMax[i] = Math.max(height[i], rightMax[i+1]);

}

int water = 0;

for(int i=1; i < len-1; i++) {

water += Math.min(leftMax[i], rightMax[i]) - height[i];

}

return water;

}

}