LeetCode 687. 最长同值路径

题目链接：https://leetcode.cn/problems/longest-univalue-path/

C++ 代码如下：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
private:
    int res;

public:
    int longestUnivaluePath(TreeNode* root) {
        res = 0;
        longest(root);
        return res;
    }

    int longest(TreeNode* root) {
        if (root == nullptr) return 0;
        int left = longest(root->left);
        int right = longest(root->right);
        
        if (root->left != nullptr && root->left->val == root->val) {
            left++;
        } else {
            left = 0;
        }

        if (root->right != nullptr && root->right->val == root->val) {
            right++;
        } else {
            right = 0;
        }

        res = max(res, left + right);

        return max(left, right);
    }
};