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Sequence of numbers in random order and 1 of them is missing how to find that out...
N(N+1)/2 - sum of the input = missing number
It isn't stated that sequence starts from 1. The sequence could be 8,9,5,6!
If the sequence is guaranteed to contain only positive integers, it can be done like so: Read in the sequence, noting the MIN and MAX numbers. The sum IF it started from 1 would be MAX(MAX-1)/2. The sum of the 'missing' numbers (from 1 up to where the sequence actually starts) is (MIN-1)*MIN/2. The missing number is given by taking the difference between the two: X = [MAX*(MAX-1) - (MIN-1)*MIN]/2.
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