def lonelyinteger(a):
    '''
    Mathematical solution
    
    The sum of the elements of a is the sum of the
    doubles of each repeated element, plus the lonely (b)
    sum(a) = sum[2.x] + b
    
    The sum of the unique elements is the sum of all
    the unique elements that are repeated, plus the lonely (b)
    sum(set(a)) = sum[x] + b
    
    Then, 2*sum(set(a)) - sum(a) = b
    
    Time complexity: O(n)
    Space complexity: O(1)
    '''
    
    return 2*sum(set(a)) - sum(a)