LeetCode-binary-tree-maximum-path-sum

Given a binary tree, find the maximum path sum.

The path may start and end at any node in the tree.

For example:
Given the below binary tree,

       1
      / \
     2   3

Return6.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    int maxSum = Integer.MIN_VALUE;
    public int maxPathSum(TreeNode root) {
        if(root == null) {
            return 0;
        }
        countPathSum(root);
        return maxSum;
    }
     
    public int countPathSum(TreeNode root) {
        if(root == null) {
            return 0;
        }
        int sum = root.val;
        int left = countPathSum(root.left);
        int right = countPathSum(root.right);
        if(left > 0) {
            sum += left;
        }
        if(right > 0) {
            sum += right;
        }
        if(maxSum < sum) {
            maxSum = sum;
        }
        return Math.max(left, right) > 0 ? root.val + Math.max(left, right) : root.val;
    }
}