二叉树的前序遍历

 1 /**
 2  * Definition of TreeNode:
 3  * class TreeNode {
 4  * public:
 5  *     int val;
 6  *     TreeNode *left, *right;
 7  *     TreeNode(int val) {
 8  *         this->val = val;
 9  *         this->left = this->right = NULL;
10  *     }
11  * }
12  */
13 
14 class Solution {
15 public:
16     vector<int> s;
17     /**
18      * @param root: The root of binary tree.
19      * @return: Preorder in vector which contains node values.
20      */
21      
22     vector<int> preorderTraversal(TreeNode *root) {
23         // write your code here
24         if(root==NULL)
25         return s;
26         if(root!=NULL)
27         {
28             s.push_back(root->val);
29             preorderTraversal(root->left);
30             preorderTraversal(root->right);
31             return s;
32         }
33     }
34 };