leetcode 543. 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

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.

 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 diameterOfBinaryTree(TreeNode* root) {
13         int ans = 1;
14         depth(root, ans);
15         return ans - 1;
16     }
17 private:
18     int depth(TreeNode *root, int &ans) {
19         if (root == NULL)
20             return 0;
21         int l = depth(root->left, ans);
22         int r = depth(root->right, ans);
23         ans = max(ans, l + r + 1);
24         return max(l, r) + 1;
25     }
26 };