144. Binary Tree Preorder Traversal

144. Binary Tree Preorder Traversal

/**
* 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) {
vector<int> list;
TreeNode* p = root;
stack<TreeNode*> st;
while (p || !st.empty()) {
if (p) {
st.push(p);
list.push_back(p->val);
p = p->left;
} else {
p = st.top();
st.pop();
p = p->right;
}
}
return list;
}
};