Trapping rain Water problem leetCode

 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

class Solution {

    public int trap(int[] height) {

        if (height == null || height.length == 0) return 0;

        int n = height.length;

        int[] leftMax = new int[n];

        int[] rightMax = new int[n];

        int water = 0;

        // Fill leftMax array

        leftMax[0] = height[0];

        for (int i = 1; i < n; i++) {

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

        }

        // Fill rightMax array

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

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

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

        }

        // Calculate trapped water

        for (int i = 0; i < n; i++) {

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

        }

        return water;

    }

}