Given a string str and an integer k, you need to rearrange the string in a way that the same characters must be at least k distance away. If not possible, return "".

Example1: 

Input: 

str = "aabb", k = 2

Output: 

You can return either "abab" or "baba". 

Example2: 

Input: 

str = "aaaa", k = 2

Output: 

return ""