Step 1: Divide the number into separate digits:
67210 => 6 7 2 1 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 7 => 1
7 to 2 => 5
2 to 1 => 1
1 to 0 => 1
0 to 6 => 6
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
5 => N
1 => C
1 => C
6 => N
Step 4: Combine the filters to a string:
C N C C N = CNCCN