二叉搜索树转为顺序双向链表

import java.util.*;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    ArrayList<TreeNode> res = new ArrayList<TreeNode>(); //作为存储数据（因为不让开新的节点）
    public TreeNode Convert(TreeNode pRootOfTree) {
        if(pRootOfTree == null) return null; //临界值非空判断
        sol(pRootOfTree);
        for(int i =0 ; i<res.size()-1;i++){ //对集合里的节点进行连接
            res.get(i).right = res.get(i+1);
            res.get(i+1).left = res.get(i);
        }
       return res.get(0);
    }
    public TreeNode sol(TreeNode root){
        if(root == null) return null; 
        TreeNode left = sol(root.left); //因为是中序遍历，所以在left和right之间进行节点的存储
        res.add(root);
        TreeNode right = sol(root.right);
       return root;
        }
    
    }