```
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
/* Morris preorder traversal O(n) time & O(1) space */
List<Integer> list = new ArrayList<>();
TreeNode curr = root;
while(curr!=null){
if(curr.left == null){
list.add(curr.val);
curr = curr.right;
}else{
TreeNode predecessor = curr.left;
while(predecessor.right!=null && predecessor.right!=curr){
predecessor = predecessor.right;
}
if(predecessor.right == null){
predecessor.right = curr;
list.add(curr.val);
curr = curr.left;
}else{
predecessor.right = null;
curr = curr.right;
}
}
}
return list;
}
}
```