目录 一、合并升序链表问题二、题目：[21. 合并两个有序链表](https://leetcode.cn/problems/merge-two-sorted-lists/description/)1、掌握dummy节点的技巧 三、题目：[23. 合并 K 个升序链表](https://leetcode.cn/problems/merge-k-sorted-lists/description/

//string 变char[]char[] chars= s.toCharArray();//char[] 变stringnew String(chars);//char 字符赋值-char首字母小写char rep='a';//rep变成brep=(char)(rep+1);//字符串切割.一定是双引号String[] sub="11001122".split("1")