Leetcode #144 Binary Tree Preorder Traversal

 1 class Solution
 2 {
 3 public:
 4     vector<int> preorderTraversal(TreeNode *root)
 5     {
 6         vector<int> output;
 7         if(root == NULL)
 8         {
 9             return output;
10         }
11 
12         stack<TreeNode*> nodeStack;
13         nodeStack.push(root);
14 
15         while(!nodeStack.empty())
16         {
17             TreeNode *topNode = nodeStack.top();
18             TreeNode *topleftNode = topNode->left, *toprightNode = topNode->right;
19             output.push_back(topNode->val);
20             nodeStack.pop(); //先pop再push。
21             
22             if(toprightNode != NULL) //因为最后访问右边的节点，所以先把右边的节点压入栈。
23                 nodeStack.push(toprightNode);
24             if(topleftNode != NULL)
25                 nodeStack.push(topleftNode);            
26         }
27 
28         return output;
29     }
30 };