蓝桥杯每日一练——二叉树的前序遍历

二叉树的前序遍历https://leetcode-cn.com/problems/binary-tree-preorder-traversal/

题目描述：

给你二叉树的根节点 root ，返回它节点值的 前序 遍历。 

class Solution {public:    void preorder(TreeNode *root, vector &res) { if (root == nullptr) {     return; } res.push_back(root->val); preorder(root->left, res); preorder(root->right, res);    }    vector preorderTraversal(TreeNode *root) { vector res; preorder(root, res); return res;    }};