Python solution to the following algorithm question: 'Least Substring

Given a string containing n lowercase letters, the string needs to divide into several substrings, the letter in the substring should be same, and the number of letters in the substring does not exceed k, and output the minimal substring number meeting the requirement.

Example

Give s = "aabbbc", k = 2, return 4

Explaination: we can get "aa", "bb", "b", "c" four substring. 

Give s = "aabbbc", k = 3, return 3

Explaination: we can get "aa", "bbb", "c" three substring.