微软面试题： LeetCode 543. 二叉树的直径出现次数：3

 1 class Solution {
 2 public:
 3     int diameterOfBinaryTree(TreeNode* root)
 4     {
 5         res = 0;
 6         int tmp = diameterOfBinaryTreeHlper(root);
 7         return res;
 8     }
 9 
10     int diameterOfBinaryTreeHlper(TreeNode* root)
11     {
12         if(root == NULL ) return 0;
13         int left = diameterOfBinaryTreeHlper(root->left);
14         int right = diameterOfBinaryTreeHlper(root->right);
15         int tree_depth = root->left == NULL && root->right == NULL ?0: max(left,right) + 1;
16         int left_depth = root->left == NULL?0:left+1;
17         int right_depth = root->right == NULL?0:right+1;
18         res = max(res,left_depth + right_depth);
19         return tree_depth;
20     }
21 private:
22     int res;
23 };