答案

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    int result=Integer.MIN_VALUE;
    int calNode(TreeNode root)
    {
       int left=root.val;
       int right=root.val;
       if(root.left!=null)
       {
           left+=calNode(root.left);
       }
       if(root.right!=null)
       {
           right+=calNode(root.right);
       }
       left=Math.max(left,root.val);
       right=Math.max(right,root.val);
       result=Math.max(result,left+right-root.val);
       return Math.max(left,right);
    }
    public int maxPathSum(TreeNode root) {
        calNode(root);
        return result;
    }
}