1. Given a big of characters, generate all kinds of permutation with length==k.
e.g: bag: {a:2, b:1}, k=2 -> {aa, ab, ba}
follow up: in the above case: ab and ba are considered as duplicated.
e.g:bag: {a:2, b:1}, k=2 -> {aa, ab}

2. Ask me to design sth like “Big Integer” in JAVA.
Finally, turn out to be “String Multiply”: Given two numbers represented as strings, return multiplication of the numbers as a string.