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）：

1 /**
2  * Definition for a binary tree node.
3  * struct TreeNode {
4  *     int val;
5  *     TreeNode *left;
6  *     TreeNode *right;
7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
8  * };
9  */
10 class Solution {
11 public:
12     int longestUnivaluePath(TreeNode* root) {
13         int res = 0 ;
14         if (root)
15             dfs(root , res) ;
16         return res ;
17
18     }
19
20     int dfs(TreeNode* root , int& res){
21         int l = root->left ? dfs(root->left , res) : 0 ;
22         int r = root->right ? dfs(root->right , res) : 0 ;
23
24         int leftEdge = (root->left && root->left->val == root->val) ? l + 1 : 0 ;
25         int rightEdge = (root->right && root->right->val == root->val) ? r + 1 : 0 ;
26         res = max(res , leftEdge + rightEdge) ;
27         return max(leftEdge , rightEdge) ;
28     }
29 };