1 struct TreeNode {
 2     int            val;
 3     TreeNode*    left;
 4     TreeNode*    right;
 5     TreeNode(int x): val(x), left(NULL),right(NULL) {}
 6 };
 7 
 8 vector<int> preorderTraversal(TreeNode *root) //非递归的前序遍历(用栈实现)
 9 {
10     if (NULL == root) {
11         cout << "tree is empty!" << endl;
12         exit(0);
13     }
14 
15     TreeNode *pre = root;
16     //TreeNode *pTemp = pre;
17     stack<TreeNode *> tree_stack;
18     vector<int> tree_vector;
19     
20     tree_stack.push(pre);
21     while (!tree_stack.empty()) {
22         TreeNode *pTemp = tree_stack.top();
23         tree_stack.pop();
24 
25         tree_vector.push_back(pTemp->val);
26         if (pTemp->right != NULL) 
27             tree_stack.push(pTemp->right);
28         if (pTemp->left != NULL)
29             tree_stack.push(pTemp->left);
30     }
31     return tree_vector;
32 }