Leetcode: Encode String with Shortest Length && G面经

Given a non-empty string, encode the string such that its encoded length is the shortest.

The encoding rule is: k[encoded_string], where the encoded_string inside the square brackets is being repeated exactly k times.

Note:
k will be a positive integer and encoded string will not be empty or have extra space.
You may assume that the input string contains only lowercase English letters. The stringempty string, encode the string