Given a positive integer N, find the smallest nonzero multiple M of N. All digits of M can only be "0" and "1"
Return M in string format.

Example:
N = 2 ==> M = "10"
N = 3 ==> M = "111"
N = 4 ==> M = "100"

Constraints:
N <= 200
length of M <= 25

Any ideas?