Given a positive integer, return its corresponding column title as appear in an Excel sheet.

For example:

1 -> A

2 -> B

3 -> C

...

26 -> Z

27 -> AA

28 -> AB

思路：

类似于进制转换，但是感觉自己写的太啰嗦了，待优化。

string convertToTitle(int n)  {    if(n<=0) return "";   map
     
      dict;   dict[0] = 'Z';   for(int i =1;i<=26;i++)   {     dict[i] ='A' + i -1;  }  string ret;  stringstream ss;    while(n>0)  {         ss<