I think it’s O(n) because I put in n = 2 to 20, so I could see the times for a 16-letter isogram, though the longest is a 17-letter word. SUBDERMATOGLYPHIC.
Anywho, looking at that graph, I reasoned the amounts of time for a 16 letter isogram:
Time for 1 n = ~ 1.25
N
Need 16 n’s, so 1.25 * 16 = 20 to do O(n).
N LOG N
n log n + 20 = 70, (50 + 20) = n log n + n
N^2
n^2 - n/2 = 256 - 20/2 = 118 for n^2
20 = O(n) = FASTEST
That’s my vote. 