42. Trapping Rain Water

 

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 

class Solution {

public int trap(int[] height) {

int left = 1;

int right = height.length - 2;

int lMax = height[left - 1];

int rMax = height[right + 1];

int res = 0;

while ( left <= right){

if(rMax <= lMax){

res += Math.max(0,rMax - height[right]);

rMax = Math.max(rMax,height[right]);

right--;

}

else{

res += Math.max(0,lMax - height[left]);

lMax = Math.max(lMax, height[left]);

left++;

}

}

return res;

}

}