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144.cpp
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/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution2 {
public:
vector<int> preorderTraversal(TreeNode* root) {
auto h = TreeNode(0, nullptr, root);
stack<TreeNode*> st({&h});
vector<int> result;
while (!st.empty()) {
root = st.top()->right;
st.pop();
while (root) {
result.push_back(root->val);
st.push(root);
root = root->left;
}
}
return result;
}
};