Given a binary tree, return the preorder traversal of its nodes' values.
Example:
Input:[1,null,2,3]
1 \ 2 / 3 Output:[1,2,3]
Follow up: Recursive solution is trivial, could you do it iteratively?
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
stack<TreeNode*> s;
vector<int> ret;
TreeNode *p=root;
while(p||!s.empty())
{
while(p)
{
s.push(p);
ret.push_back(p->val);
p=p->left;
}
if(!s.empty())
{
p=s.top();
s.pop();
p=p->right;
}
}
return ret;
}
};
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
stack<TreeNode*> s;
vector<int> ret;
TreeNode *p=root;
while(p||!s.empty())
{
while(p)
{
s.push(p);
ret.push_back(p->val);
p=p->left;
}
if(!s.empty())
{
p=s.top();
s.pop();
p=p->right;
}
}
return ret;
}
};