Suppose you are given an integer array A that contains each number between 1 and n except for one missing number (thus A.length == n - 1.) Since 1 + 2 + ... + n = n(n+1)/2, you can find the missing with a simple subtraction: missingNumber = n(n+1)/2 - (A[0] + A[1] + ... + A[n-2]). I'll leave it to you to write the actual code!