def f(combination, count_zero, count_nonzero, added_nonzero, N):

    if count_zero + count_nonzero == 2*N:
        print(combination)
        return

    if count_zero < N:
        f(combination+'0', count_zero + 1, count_nonzero, added_nonzero, N)

    if count_nonzero < N:
        for number in range(1, N+1):
            if number not in added_nonzero:
                f(combination+str(number), count_zero, count_nonzero + 1, added_nonzero+[number], N)

f(combination = '', count_zero = 0, count_nonzero = 0, added_nonzero = [], N = 2)