Given a string size of n and print all possible string of size of r
e.g. given [a, b, c] and print all string size of 2
output: ab, ac, bc
\[ \text{Binomial Identity} \]
\[ \binom{n}{k} = \binom{n-1}{k-1} + \binom{n-1}{k} \]
\[ \binom{n}{0} = 1 \text{ with } 1 \leq k \leq n\]
