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 

          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 {
public:
    int maxLength = 0;  
    int diameterOfBinaryTree(TreeNode* root) {
        help(root);
        return maxLength;
    }
    int help(TreeNode* root){  
        if(root == NULL) return 0;  
        int left = help(root->left);  
        int right = help(root->right);  
          
        maxLength = max(left + right, maxLength);  
          
        return max(left + 1, right + 1);  
    }  
};