Diameter of Binary Tree

Given a binary tree, you need to compute the length of the diameter of the tree. The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

Example:
Given a binary tree
1
/ \
2 3
/ \
4 5
Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].
Note: The length of path between two nodes is represented by the number of edges between them.

/** * 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 {private:    int max_diameter(TreeNode* root,int &diameter){        if(!root)            return 0;        int left = max_diameter(root->left,diameter);        int right = max_diameter(root->right,diameter);        if(left + right > diameter){            diameter = left + right ;        }        return 1 + max(left,right);    }public:    int diameterOfBinaryTree(TreeNode* root) {        int res = 0;        max_diameter(root,res);        return res;    }};