LeetCode 543. 二叉树的直径

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int res=0;
    int diameterOfBinaryTree(TreeNode* root) {
        dfs(root);
        return res-1;
    }
    int dfs(TreeNode* u)
    {
        if(!u)  return 0;
        int l=dfs(u->left),r=dfs(u->right),num=0;
        res=max(res,l+r+1);
        num=max(l,r);
        num++;
        return num;
    }
};