LeetCode：144_Binary Tree Preorder Traversal | 二叉树的前序遍历 | Medium

题目：Binary Tree Preorder Traversal

二叉树的前序遍历，同样使用栈来解，代码如下：

 struct TreeNode {
     int            val;
     TreeNode*    left;
     TreeNode*    right;
     TreeNode(int x): val(x), left(NULL),right(NULL) {}
 };

 vector<int> preorderTraversal(TreeNode *root) //非递归的前序遍历(用栈实现)
 {
     if (NULL == root) {
         cout << "tree is empty!" << endl;
         exit();
     }

     TreeNode *pre = root;
     //TreeNode *pTemp = pre;
     stack<TreeNode *> tree_stack;
     vector<int> tree_vector;

     tree_stack.push(pre);
     while (!tree_stack.empty()) {
         TreeNode *pTemp = tree_stack.top();
         tree_stack.pop();

         tree_vector.push_back(pTemp->val);
         if (pTemp->right != NULL)
             tree_stack.push(pTemp->right);
         if (pTemp->left != NULL)
             tree_stack.push(pTemp->left);
     }
     return tree_vector;
 }