morris preorder.java

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