leetcode 543. 二叉树的直径

	/**
 * 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 dfs(TreeNode *root)
    {
      if(!root)return 0;
      int left=dfs(root->left);
      int right=dfs(root->right);
      ans=max(ans,left+right);//两个节点构成一条路径，当前root与左右最大节点个数
      return max(left,right)+1;
      
    }
    int diameterOfBinaryTree(TreeNode* root) {
       dfs(root);
       return ans;
    }
};