elements = [1,2,3,4,5,6]
combinations = []
k = 3
 
def get_combinations_BFS():
    queue = [([],0)]
    while queue:
        combination, elem_idx = queue.pop()
 
        if elem_idx == len(elements):
            continue
 
        new_comb = combination[:]
        new_comb.append(elements[elem_idx])
 
        if len(new_comb) == k:
            combinations.append(new_comb)
 
        queue.append((combination, elem_idx+1))
        queue.append((new_comb, elem_idx+1))
 
get_combinations_BFS()
 
 
print(combinations)

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