LeetCode——二叉树的前序遍历

题目地址：https://leetcode-cn.com/problems/binary-tree-preorder-traversal/

解题思路：树构造以及递归遍历

class Solution {
    vector<int> ans;
public:
    vector<int> preorderTraversal(TreeNode* root) {
        if (root) {
            ans.push_back(root->val);
            preorderTraversal(root->left);
            preorderTraversal(root->right);
        }
        return ans;
            
    }
};