[LeetCode] Nonrecursive preorder traversal

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]

.
下面给出非递归前序遍历的算法，和非递归中序遍历非常像。

vector<int> preorderTraversal(TreeNode *root) {
vector<int> vec; // stores the preorder sequence
stack<TreeNode*> st; // auxiliary stack
TreeNode* curr = root;

while(1)
{
while(curr)
{
vec.push_back(curr->val);
st.push(curr);
curr = curr->left;
}

if(st.size()==0)
break;
else
{
curr = st.top();
curr = curr->right;
st.pop();
}
}

return vec;
}