Repeatedly take n elements from a list

Suppose I have a list

lis={a,b,c,d,e,f};

Is there a way to get all the combination of 3 elements (without order) from it? I try to use

Flatten[Permutations /@ Subsets[lis, {3}], 1]

But it not contains the repeated elements, that is I want the elements such as {a,a,a} be included also.

Answer

Using Subsets you can avoid deleting many duplicates

Multisets[s_List, n_Integer] := With[{u = Range[0, n - 1]},

        s[[# - u]] & /@ Subsets[Range[Length[s] + n - 1], {n}]]
Multisets[{a, b, c, d, e}, 3]

This vectorized version is probably faster:

Multisets[s_List, n_Integer] := Partition[s[[Flatten[Transpose[Subsets[
    Range[Length[s] + n - 1], {n}]] - Range[0, n - 1], {2, 1}]]], n]

Comments

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