/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    int longestUnivaluePath(TreeNode* root) {        int res = 0;        helper(root, res);        return res;    }    int helper(TreeNode* root, int &res) {        if (root == NULL)   return 0;        int ret = 0;        int left = helper(root->left, res);        int right = helper(root->right, res);        if (root->left && root->left->val == root->val) {            res = max(res, left + 1);            ret = left + 1;        }        if (root->right && root->right->val == root->val) {            res = max(res, right + 1);            ret = max(ret, right + 1);        }        if (root->left && root->left->val == root->val &&           root->right && root->right->val == root->val)            res = max(res, left + right + 2);        return ret;    }};