Binary Tree Loop Solution - Flatten Binary Tree to Linked List

Get preOrder List, then reset the pointer

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void flatten(TreeNode root) {
        if(root == null){
            return;
        }
       
        TreeNode curNode = root;
        Queue<TreeNode> processed = new LinkedList<TreeNode>();
        Stack<TreeNode> process = new Stack<TreeNode>();
        process.push(root);
        processed.offer(root);
        while(!process.isEmpty() || curNode != null){
            if(curNode != null){
                process.push(curNode);
                processed.offer(curNode);
                curNode = curNode.left;
            }else{
                curNode = process.pop();
                curNode = curNode.right;
            }
        }
       
        Iterator it = processed.iterator();
        TreeNode lastNode = root;
        if(it.hasNext()){
            lastNode = (TreeNode) it.next();
        }
        while(it.hasNext()){
            TreeNode cur = (TreeNode) it.next();
            lastNode.right = cur;
            lastNode.left = null;
            lastNode = cur;
        }
        lastNode.right = null;
        lastNode.left = null;
    }
}