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.

思路：

求與當前結點相同的路徑長度的時候，類似於先序遍歷。

 int longPath(TreeNode* root,int pre)//與當前結點相同的最長路徑長度
 {
   
 
if(root == NULL ||  root->val != pre) return 0;
   int left = longPath(root->left,root->val);
   int right = longPath(root->right,root->val);
   return 1+max(left,right);      
}
 int longestUnivaluePath(TreeNode* root)
 {
    int ret = 0;
    if(root == NULL) return 0;
    int left = longestUnivaluePath(root ->left);
    
 
int right = longestUnivaluePath(root->right);
    ret = max(max(left,right),longPath(root->left,root->val)+longPath(root->right,root->val));
    return ret;
 }

[leetcode-687-Longest Univalue Path]