Given a non-empty list of integers named integer_list, where every element appears exactly twice except for one element which appears only once, return the element that appears only once. The solution must have linear runtime complexity and use only constant extra space.
Example 1
Input: [7, 3, 3]
Output: 7
Explanation: 7 appears once, 3 appears twice.
Example 2
Input: [10, 10, 5, 5, 8]
Output: 8
Explanation: 8 appears once, others appear twice.
Example 3
Input: [42]
Output: 42
Explanation: Only one element, which is unique.
Constraints
Case 1
Input: [6, 9, 6]
Expected: 9
Case 2
Input: [12, 15, 12, 18, 15]
Expected: 18
Case 3
Input: [100, 200, 100]
Expected: 200
Case 4
Input: [0, 0, -1]
Expected: -1
Case 5
Input: [23, 23, 45, 45, 67]
Expected: 67