543. Diameter of Binary Tree（LeetCode）

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