687.cpp

/**
 * 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;
        if (root) dfs(root, res);
        return res;
    }
    int dfs(TreeNode* node, int &res) {
        int l = node->left ? dfs(node->left, res) : 0;
        int r = node->right ? dfs(node->right, res) : 0;
        int resl = node->left && node->left->val == node->val ? l + 1 : 0;
        int resr = node->right && node->right->val == node->val ? r + 1 : 0;
        res = max(res, resl + resr);
        return max(resl, resr);
    }
};