687. Longest Univalue Path

Given a binary tree, find the length of the longest path where each node in the path has the same value. This path may or may not pass through the root.

Note: The length of path between two nodes is represented by the number of edges between them.

Example 1:

Input:

              5
             / 
            4   5
           /    
          1   1   5

 

Output:

2

 

Example 2:

Input:

              1
             / 
            4   5
           /    
          4   4   5

 

Output:

2

 

Note: The given binary tree has not more than 10000 nodes. The height of the tree is not more than 1000.

找出相同节点相连的路径，求最长的边数和

C++（71ms）：

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