SAP Ariba interview question

Write a Linked List in Java. Write a program to use the Linked List to implement a Stack in Java. Write a program to print all permutations and combinations of possible words from a given String (no use of any libraries or apis) in java eg: For String abc, result should be a, ab, ac, bc, ba, ca, cb, acb, cba, bac, bca etc.input string can be of any length.