144. Binary Tree Preorder Traversal

Not much to say. Pretty straightforward.

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:      stack<TreeNode *> stk;  
14:      vector<int> res;  
15:      if (root == NULL) return res;  
16:      stk.push(root);  
17:      while (!stk.empty()) {  
18:        TreeNode *t = stk.top();  
19:        stk.pop();  
20:        res.push_back(t->val);  
21:        if (t->right) stk.push(t->right);  
22:        if (t->left) stk.push(t->left);  
23:      }  
24:      return res;  
25:    }  
26:  };