class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
Stack<TreeNode> stack = new Stack<>();
List<Integer> list = new ArrayList<>();
TreeNode cur = root;
TreeNode top = null;
while (cur != null || !stack.empty()){
while (cur != null){
System.out.print(cur.val+" ");
list.add(cur.val);
stack.push(cur);
cur = cur.left;
}
top = stack.pop();
cur = top.right;
}
return list;
}
}
