114. Flatten Binary Tree to Linked List

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void flatten(TreeNode root) {
        //二叉树的先序遍历，并且将结果以链表的形式输出
        preOrder(root);
        
        for(int i=0;i<tree.size();i++)
        {
            if(i==tree.size()-1)
            {
                //尾
                tree.get(i).left=null;
                tree.get(i).right=null;
            }
            else
            {
                tree.get(i).left=null;
                tree.get(i).right=tree.get(i+1);
            }
        }
 
        
        
    }
    
    
    public static List<TreeNode> tree=new ArrayList<TreeNode>();
    
    public void preOrder(TreeNode root)
    {
        if(root==null)
            return ;
        //访问节点 
        tree.add(root);
        //递归左子
        preOrder(root.left);
        //递归右子
        preOrder(root.right);
    }
    
}