解题思路

class Solution {
    int res = Integer.MIN_VALUE;
    public int maxPathSum(TreeNode root) {
        helper(root);
        return res;
    }
    private int helper(TreeNode root) {
        if (root == null) return 0;
        //左边最大值
        int left = helper(root.left);
        //右边最大值
        int right = helper(root.right);
        //和全局变量比较
        res = Math.max(left + right + root.val, res);
        // >0 说明都能使路径变大
        return Math.max(0, Math.max(left, right) + root.val);
    }
}