Solution.cpp

/**
 * 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 {
public:
    int longestUnivaluePath(TreeNode* root) {
        int ans = 0;
        auto dfs = [&](auto&& dfs, TreeNode* root) -> int {
            if (!root) {
                return 0;
            }
            int l = dfs(dfs, root->left);
            int r = dfs(dfs, root->right);
            l = root->left && root->left->val == root->val ? l + 1 : 0;
            r = root->right && root->right->val == root->val ? r + 1 : 0;
            ans = max(ans, l + r);
            return max(l, r);
        };
        dfs(dfs, root);
        return ans;
    }
};