Diameter of Binary Tree

题目描述：

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.

思路：

后序遍历，函数int postorder(TreeNode* node, int& res)中，res存放遍历过程中最长路径，返回值是到大当前节点的最长路径。

代码：

class Solution {
public:
    int diameterOfBinaryTree(TreeNode* root) {
        if (root==NULL)
        return 0;
        int res=0;
        postorder(root,res);
        return res-1;
    }
    
    int postorder(TreeNode* node, int& res)
    {
        if (node==NULL)
            return 0;
        int maxl=postorder(node->left,res);
        int maxr=postorder(node->right, res);
        if (res<maxl+maxr+1)
            res=maxl+maxr+1;
        return max(maxl,maxr)+1; 
    }
};