二叉树的前序遍历

给出一棵二叉树，返回其节点值的前序遍历。

您在真实的面试中是否遇到过这个题？ 

Yes

样例

给出一棵二叉树 {1,#,2,3},

   1    \     2    /   3

 返回 [1,2,3].

解题思路：dfs遍历前序保存

class Solution {public:    /**     * @param root: The root of binary tree.     * @return: Preorder in vector which contains node values.     */    vector<int>ans;    vector<int> preorderTraversal(TreeNode *root) {        // write your code here        if(root==NULL)return ans;        else ans.push_back(root->val);        preorderTraversal(root->left);        preorderTraversal(root->right);        return ans;    }};