Refer to the http://hmkcode.github.io/calculate-find-all-possible-combinations-of-an-array-using-java/ for more info.

Given an array of size N e.g. e={'A','B','C','D','E'} N=5, we want to find all possible combinations of K elements in that array. For example, if K=3 then one possible combination is of array e is `{'A','B','C'}. Here we have three different algorithms for finding k-combinations of an array.