While the hash-based approach (using a frequency map or set) is intuitive, the constraint of O(1) space suggests using Bit Manipulation (XOR). Property of XOR: a ^ a = 0 and a ^ 0 = a.

1. Initialize res = 0.
2. For every num in nums:
   • res ^= num.
3. Return res.

• Time Complexity: O(N).
• Space Complexity: O(1).

def single_number(nums):
    res = 0
    for n in nums:
        res ^= n
    return res