def dfs(elements):
        # 리프 노드일때 결과 추가
        if len(elements) == 0:
            results.append(prev_elements[:])

        # 순열 생성 재귀 호출
        for e in elements:
            next_elements = elements[:]
            next_elements.remove(e)

            prev_elements.append(e)
            dfs(next_elements)
            prev_elements.pop()