BinaryTreePreorderTraversal.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class BinaryTreePreorderTraversal {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        TreeNode current = root;
        while (current != null) {
            if (current.left == null) {
                res.add(current.val);
                current = current.right;
            } else {
                TreeNode prev = current.left;
                while (prev.right != null && prev.right != current)
                    prev = prev.right;
                if (prev.right == null) {
                    prev.right = current;
                    res.add(current.val);
                    current = current.left;
                } else {
                    prev.right = null;
                    current = current.right;
                }
            }
        }
        return res;
    }
}