LeetCode -- Flatten 二叉树

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);
		}
	}
    
}