/**
 * 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> a;
        stack<TreeNode*> s;
        TreeNode* cur = root;
        while(!s.empty() || cur!=NULL){
            if(cur!=NULL){
                a.push_back(cur->val);
                s.push(cur);
                cur = cur->left;
            }else{
                cur = s.top();
                s.pop();
                cur = cur->right;
            }
        }
        return a;
    }
};

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> s = new Stack<>();
        TreeNode cur = root;
        while(cur!=null || !s.isEmpty()){
            if(cur!=null){
                list.add(cur.val);
                s.push(cur);
                cur = cur.left;
            }else{
                cur = s.pop();
                cur = cur.right;
            }
        }
        return list;
    }
}