Bijections are fun to look at and amusing especially to cryptographers, can
you spot out one here?
Python
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from secret import flag
def nk2n(nk):
l = len(nk)
if l==1:
return nk[0]
elif l==2:
i,j = nk
return ((i+j)*(i+j+1))//2 +j
return nk2n([nk2n(nk[:l-l//2]), nk2n(nk[l-l//2:])])
print(nk2n(flag))
# output
# 1066464516621568650416778516260128065562999836454777449496486613730252783905\
# 58656231590803591166516524516182041583860744711996793449978222571578932566\
# 51539240517205572748689616288529831032342817805470118893063573639935906790\
# 19094216260987077393364474718427466510193852