687

/**
 * 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 maxLen = 0;
    int longestUnivaluePath(TreeNode* root) {
        dfs(root);
        return maxLen;
    }

    int dfs(TreeNode* root){
        int len1 = 0, len2 = 0;
        if(!root) return 0;
        if(root->left){
            int left = dfs(root->left);
            if(root->val == root->left->val){
                len1 = left+1;
            }
        }
        if(root->right){
            int right = dfs(root->right);
            if(root->val == root->right->val){
                len2 = right+1;
            }
        }
        maxLen = max(len1+len2, maxLen);
        return max(len1, len2);
    }
};