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) //非递归的中序遍历(用栈实现)
 9 {
10     if (NULL == root) {
11         return vector<int>();
12     }
13 
14     stack<TreeNode *> tree_stack;
15     vector<int> tree_vector;
16 
17     TreeNode *pTemp = root;
18     while (pTemp || !tree_stack.empty()) {
19         while (pTemp) {
20             tree_stack.push(pTemp);
21             pTemp = pTemp->left;
22         }
23         if (!tree_stack.empty()) {
24             pTemp = tree_stack.top();
25             tree_vector.push_back(pTemp->val);
26             tree_stack.pop();
27             pTemp = pTemp->right;
28         }
29     }
30     return tree_vector;
31 }