1. 先序遍历
2. 节点用队列存储
3. 遍历队列，建立链表

实现：

public class Solution {

public void Flatten(TreeNode root)

 {

     if(root == null)

	 {

		return;

	 }

	 Travel(root);

	 root = _nodes[0];

	 root.left = null;

	 root.right = null;

	 for(var i = 1;i < _nodes.Count; i++){

			var l = _nodes[i];

			l.left = null;

			l.right = null;

			root.right = l;

			root = root.right;

	 }

 }

	private List<TreeNode> _nodes = new List<TreeNode>();

	private void Travel(TreeNode root){

		_nodes.Add(root);

		if(root.left != null){

			Travel(root.left);

		}

		if(root.right != null){

			Travel(root.right);

		}

	}

}