144. 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 {
12     vector<int> res;
13 public:
14     vector<int> preorderTraversal(TreeNode* root) 
15     {
16         if(!root) return res;
17         stack<TreeNode*> s;
18         s.push(root);
19         while(!s.empty())
20         {
21             root = s.top();
22             s.pop();
23             res.push_back(root->val);
24             if(root->right) s.push(root->right);
25             if(root->left) s.push(root->left);
26         }
27         return res;
28     }
29 };