Update SingleNumber Implementation to Use Bitwise XOR

[harshendram]

This is a(n):

• [ ] New solution
• [x] Update to an existing solution
• [ ] Error

Details: The current implementation of the singleNumber function uses a map to count occurrences of each number in the vector. This approach has a time complexity of O(n) and a space complexity of O(n) due to the storage requirements of the map.It also helps competitive programmers to reduce time.

I propose updating the implementation to use the XOR bitwise operation, which provides a more efficient solution. The XOR approach has a time complexity of O(n) but reduces the space complexity to O(1), as it does not require any additional data structures.