Single Number
Updated Jan 31, 2026
Problem
Given a non-empty array of integers nums, every element appears twice except for one. Find that single one.
You must implement a solution with a linear runtime complexity and use only constant extra space.
Constraints
1 <= nums.length <= 3 * 10^4
-3 * 10^4 <= nums[i] <= 3 * 10^4
Each element in the array appears twice except for one element which appears only once.
Examples
Example 1
Input:
nums = [2,2,1]
Output:
1
Example 2
Input:
nums = [4,1,2,1,2]
Output:
4
Example 3
Input:
nums = [1]
Output:
1
Function Signature
def singleNumber(self, nums: list[int]) -> int
How to Submit
Implement a Solution class with a singleNumber method.
Your method will be called with the input parameters and should return the answer.