二叉树前序遍历

class Solution {
public:
   
    void preorder(vector&res,TreeNode*root)//读取树内的值
    {
        if(root==NULL)
        {
            return ;
        }
        res.push_back(root->val);//本节点内的元素放到容器vector中
        preorder(res,root->left);//指向左节点，进行读取
        preorder(res,root->right);//指向右节点，进行读取
    }
    vector preorderTraversal(TreeNode* root) {//传入一个树的指针，调用preorder
        vectorres;
        preorder(res,root);
        return res;
        // write code here
    }
};