牛客JZ36——二叉搜索树与双向链表（JAVA）

题目描述：
https://www.nowcoder.com/practice/947f6eb80d944a84850b0538bf0ec3a5

思路：双向链表的结构其实就是中序遍历，可以先将数据转为中序遍历的数组，最后在遍历数组，新建链表
而树中的right就可以看做是双向链表中的next，而left就可以看做是双向链表的pre

import java.util.ArrayList;

/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

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

    }

}
*/
// 中序遍历
public class Solution {
    TreeNode res=null;
    ArrayList<Integer> list=new ArrayList<>();
    ArrayList order(TreeNode pRootOfTree){
        if(pRootOfTree==null){
            return list;
        }
        order(pRootOfTree.left);
        list.add(pRootOfTree.val);
        order(pRootOfTree.right);
        return list;
    }

    public TreeNode Convert(TreeNode pRootOfTree) {
        if(pRootOfTree==null){
            return pRootOfTree;
        }
        order(pRootOfTree);
        int n=list.size();
        TreeNode head=new TreeNode(list.get(0));
        res=head;
        for(int i=1;i<n;i++){
            TreeNode temp=new TreeNode(list.get(i));
            head.right=temp;
            head=head.right;
        }
        for(int i=n-2;i>=0;i--){
            head.left=new TreeNode(list.get(i));
            head=head.left;
        }
        return res;
    }
}