Leetcode 543. Diameter of Binary Tree（二叉树直径）

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]. 

/**
 * 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 ans=0;
    int anothergetlength(TreeNode*root){
     
        if(root==NULL){
     
            return 0;
        }
        int left=anothergetlength(root->left);
        int right=anothergetlength(root->right);
        ans=max(ans,left+right);
        return left>right?(left+1):(right+1);
    }
    int diameterOfBinaryTree(TreeNode* root) {
     
        anothergetlength(root);
        return ans;
    }
};

这个问题巧妙在private那里
1，首先要明确最长的路径并不一定过根节点，自己想的那种求左右子树之和加起来就不行了
2，其次要在private函数里求出各个子树的长度，其实在平常的求长度那里也只这么算的，但之前只是用来求最大长度就扔了，现在我们用一行代码把这些信息利用起来
3，此外，路径数==节点数-1
4,ans要定义为全局变量

ans = max(ans, left+right + 1);//这个语句实际上就实现了求最大值
//若最长路径在左子树中而不经过根节点，这条语句可以挑出这个最大值