LeetCode Binary Tree Preorder Traversal (二叉树先序遍历非递归)

题目要求：

Given a binary tree, return the preorder traversal of its nodes' values.

For example:

Given binary tree

{1,#,2,3}

,

1
\
2
/
3

return

[1,2,3]

.

Note: Recursive solution is trivial, could you do it iteratively?
代码：

class Solution {
public:
vector<int> preorderTraversal(TreeNode *root) {

vector<int> ret;
if(root == NULL)
return ret;
stack<TreeNode*> st;
st.push(root);
TreeNode* cur = NULL;
while(!st.empty())
{
cur = st.top();
ret.push_back(cur->val);
st.pop();
if(cur->right != NULL)
st.push(cur->right);
if(cur->left != NULL)
st.push(cur->left);
}
return ret;

}
};