leetcode 144. Binary Tree Preorder Traversal 二叉树的前序遍历

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<int> preorderTraversal(TreeNode* root) {
13         //二叉树前序遍历的递归写法，
14         vector<int> res;
15         preorder(root,res);
16         return res;
17     }
18     void preorder(TreeNode* root,vector<int> &res){
19         if(root==NULL) return;
20         res.push_back(root->val);
21         if(root->left) preorder(root->left,res);
22         if(root->right) preorder(root->right,res);
23     }
24 };