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;
}
};