串的基本操作

设串中元素为字符型数据，编写函数实现以下操作：

（1）求子串：给定一个字符串s，求其中从指定位置start开始的，长度为len的子串，并返回。例如：s =  abcdefg ，start = 2，len = 3，则截取的子串为 cde 。

（2）删除子串：给定字符串s，从第pos位开始，删除长度为len的子串。例如：s =  abcdefg ，start = 2，len = 3，则删除后，s变为为 abfg 。

（3）插入子串：给定两个字符串s1和s2，及指定位置pos，将s2插入到s1的第pos之后。例如：s1 =  abcdefg ，s2 =  xyz ，pos=3，插入完成后，s1变为 abcdxyzefg 。

（4）串置换：给定字符串s1和s2，及指定位置pos，将s1中从第pos位开始的一段子串(长度与s2相等)用s2替换。例如：s1 =  abcdefg ，s2 =  xyz ，pos=3，插入完成后，s1变为 abcxyzg 。

-Basic operation string

Let the elements of character string data, write a function to achieve the following:

(1) her series: Given a string s, find which start from the beginning of the specified location, the length of len substring and returns. For example: s = 　abcdefg  , start = 2, len = 3, then intercepted substring 　cde  .

(2) Delete substring: Given a string s, beginning from the first position pos, delete substring of length len. For example: s = 　abcdefg  , start = 2, len = 3, then deleted, s becomes as 　abfg  .

(3) insert substring: Given two strings s1 and s2, and specify the location pos, s2 inserted after the first pos s1 s. For example: s1 = 　abcdefg  , s2 = 　xyz  , pos = 3, after the completion of insertion, s1 becomes 　abcdxyzefg  .

(4) string substitution: Given strings s1 and s2, and specify the location pos, will be in s1 (s2 equal length) from the pos-b