Given "abcabcbb", the answer is "abc", which the length is 3. Given "bbbbb", the answer is "b", with the length of 1. Given "pwwkew", the answer is "wke", with the ...

Given a string S with only lowercase characters. Return the number of substrings that contains at least k distinct characters. Explanation: Any substring whose length is not smaller than 3 contains a, ...