flatten - Java solution - study the code written using Queue

flatten3A.java

/**
 * Definition for binary tree
 * class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
	public TreeNode flatten(TreeNode A) {
	    
	    Queue<TreeNode> queue = new LinkedList<TreeNode>();
	    preorder(queue, A);
	    
	    if (queue.isEmpty())
	        return A;
	    
	    TreeNode node = queue.remove();
	    TreeNode next;
	    A = node;
	    
	    while (!queue.isEmpty()) {
	        next = queue.remove();
	        node.left = null;
	        node.right = next;
	        next.left = null;
	        node = next;
	    }
	    
	    return A;
	    
	}
	
	public void preorder(Queue<TreeNode> queue, TreeNode node) {
	    
	    if (node == null)
	        return;
	    
	    queue.add(node);
	    preorder(queue, node.left);
	    preorder(queue, node.right);
	    
	}
	
}