PreorderSolution.java

/// Source : https://leetcode.com/problems/binary-tree-preorder-traversal/description/
/// Author : liuyubobobo
/// Time   : 2018-05-29

import java.util.ArrayList;
import java.util.List;

// PreOrder Morris Traversal
// Time Complexity: O(n), n is the node number in the tree
// Space Complexity: O(1)
public class PreorderSolution {

    public List<Integer> preorderTraversal(TreeNode root) {

        ArrayList<Integer> res = new ArrayList<Integer>();
        if(root == null)
            return res;

        TreeNode cur = root;
        while(cur != null){
            if(cur.left == null){
                res.add(cur.val);
                cur = cur.right;
            }
            else{
                TreeNode prev = cur.left;
                while(prev.right != null && prev.right != cur)
                    prev = prev.right;

                if(prev.right == null){
                    res.add(cur.val);
                    prev.right = cur;
                    cur = cur.left;
                }
                else{
                    prev.right = null;
                    cur = cur.right;
                }
            }
        }

        return res;
    }
}