Accepts an array of integers from 1-N in which one number is missing. N is equal to the length of the array minus 1.

FindMissingNumber.groovy

#!/usr/local/bin/groovy

def findMissingNumber(def a) {
	
	lowerSum = a.inject(0, {
		sum, it -> sum += it
	})
	upperSum = (1..(a.size + 1)).inject(0, {
		sum, it -> sum += it 
	})
	return upperSum - lowerSum
}

assert findMissingNumber([1, 2, 3, 5]) == 4
assert findMissingNumber([4, 3, 5, 2]) == 1