#Strings

Write a Java program to generate and print all permutations of a given string.

Given two strings, S and T, find the minimum window in S which will contain all the characters of T. If there is no such window, return the empty string. Example: Input: S = 'ADOBECODEBANC', T = 'ABC'. Output: 'BANC'.

Given a string 's' and an integer 'k', return the length of the longest substring of 's' such that the frequency of each character in this substring is greater than or equal to 'k'.

Given a string `s`, find the length of the longest substring without repeating characters.

Given a list of strings, find the most frequent element and store it in the most appropriate data structure. Consider the efficiency of your solution.