42. Trapping Rain Water LeetCode Java Solution

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
• 1 <= n <= 2 * 10⁴
• 0 <= height[i] <= 10⁵

Solution

class Solution {
    public int trap(int[] height) {
        return trapping(height);
    }
        public static int trapping(int[] arr) 
    {
        int n=arr.length;
        int[] left=new int[n];
        left[0]=arr[0];
        for(int i=1;i<n;i++)
        {
            left[i]=Math.max(left[i-1],arr[i]);
        }
        int[] right=new int [n];
        right[n-1]=arr[n-1];
        for(int i=n-2;i>=0;i--)
        {
            right[i]=Math.max(right[i+1],arr[i]);
        }
        int sum=0;
        for(int i=0;i<n;i++)
        {
            sum=sum+Math.min(left[i],right[i])-arr[i];
        }
        return sum;
    }
        
    }