42. Trapping Rain Water (Hard)

/**
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
0 <= n <= 3 * 104
0 <= height[i] <= 105
*/

class Solution {
    public int trap(int[] arr) {
        int res=0;
        for(int i=1; i<arr.length-1; i++)
        { 
            int left=arr[i];
            int right=arr[i];
           
               for(int j=0; j<i; j++)
                   left=Math.max(left, arr[j]);
            for(int j = i + 1; j <arr.length ; j++)
        {
            right = Math.max(right, arr[j]);
        }
            
       res += Math.min(left, right) - arr[i];
    }
    return res;
    }
}