single-number

 # find non repeated element
        
        # soln 1 - O(n)
        # put + remove from set
        
        temp_set = set()
        
        for element in nums:
            if element not in temp_set:
                temp_set.add(element)
            else:
                temp_set.remove(element)

        return temp_set.pop()

        # bit manipulation O(1) space
        # a XOR 0 = a
        # a XoR a = 0
        
        a = 0
        for element in nums:
            a ^= element;
        return a