Given a non-empty list of integers integer_list, where every element appears exactly twice except for one element that appears only once, return the element that appears only once. The solution must run in linear time and use only constant extra space.
Example 1
Input: [7, 3, 7]
Output: 3
Explanation: 3 appears only once, 7 appears twice.
Example 2
Input: [10, 10, 5, 5, 8]
Output: 8
Explanation: 8 appears only once, 10 and 5 appear twice.
Example 3
Input: [0, 1, 1]
Output: 0
Explanation: 0 appears only once, 1 appears twice.
Constraints
Case 1
Input: [6, 6, 9]
Expected: 9
Case 2
Input: [12, 15, 12, 15, 20]
Expected: 20
Case 3
Input: [100, 200, 100]
Expected: 200
Case 4
Input: [-5, -5, 0]
Expected: 0
Case 5
Input: [42]
Expected: 42