Trapping Rain Water

class Solution {
    public int maxWater(int arr[]) {
        int n = arr.length;
        int pre[] = new int[n];
        int post[] = new int[n];
        pre[0] = arr[0];
        int ans = 0;
        post[n - 1] = arr[n - 1];
        for (int i = 1; i < n; i++)
            pre[i] = Math.max(pre[i - 1], arr[i]);

        for (int j = n - 2; j >= 0; j--)
            post[j] = Math.max(post[j + 1], arr[j]);

        for (int i = 0; i < n; i++)
            ans += Math.min(pre[i], post[i]) - arr[i];
        
        return ans;
    }
}