Typical Code Interview Problem 2: Find the single number in an array of integers

Question:

Given an array of integers, arr, where all numbers occur twice except one number which occurs once, find the number. Your solution should ideally be O(n) time and use constant extra space. Example: Input: arr = [7, 3, 5, 5, 4, 3, 4, 8, 8] Output: 7

Response from ChatGPT

class Solution(object):
    def findSingle(self, nums):
        # initialize the result variable to 0
        res = 0
        
        # iterate over each number in the array
        for num in nums:
            # perform bitwise XOR operation with the current number
            res ^= num
        
        # return the result variable, which will contain the single number
        return res

# example usage
nums = [7, 3, 5, 5, 4, 3, 4, 8, 8]
print(Solution().findSingle(nums))  # prints 7

Here’s an explanation of how the algorithm works:

1. We initialize a variable called res to 0. This variable will be used to store the result of performing the bitwise XOR operation on all the numbers in the array.
2. We iterate over each number in the array, and for each number, we perform the bitwise XOR operation with the current value of res. This has the effect of cancelling out all the numbers that occur twice, leaving only the single number that occurs once.
3. Finally, we return the value of res, which should contain the single number that occurs once.

This solution has a time complexity of O(n) and a space complexity of O(1), since it uses only a single variable to store the result.