Step 1: Divide the number into separate digits:
72897 => 7 2 8 9 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 2 => 5
2 to 8 => 6
8 to 9 => 1
9 to 7 => 2
7 to 7 => 0
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
5 => N
6 => N
1 => C
2 => N
0 => E
Step 4: Combine the filters to a string:
N N C N E = NNCNE