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

Return 6.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    int ERROR = -10000;
    int max;
    public int maxPathSum(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        max = ERROR;
        if(root == null) return 0;
        int ret = maxPathSumHelper(root);
        if(ret > max) max = ret;
        return max;
    }
    
    public int maxPathSumHelper(TreeNode root) {
        if(root == null) return ERROR;    
        int left = maxPathSumHelper(root.left);
        int right = maxPathSumHelper(root.right);
        // find out the max sum
        if(max < left ) max = left;
        if(max < right) max = right;
        if(max < (root.val + left + right)) max = root.val + left + right;
        
        return Math.max(root.val, Math.max(root.val + left, root.val + right));
    }
}