687. Longest Univalue Path

/**
 * 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;
    }
};