Single Number

Problem

Given an integer array nums where every element appears twice except for one, find that one. O(n) time, O(1) space.

Constraints

• 1 ≤ nums.length ≤ 3·10⁴
• −3·10⁴ ≤ nums[i] ≤ 3·10⁴

Approach — Core Patterns

This is a Core Patterns problem. The idea: pick the data structure best matched to the constraints and solve it in one clean pass. Work through the reference code below line by line, then re-derive it yourself in the editor — that's how the pattern sticks.

Complexity: see the walkthrough below.

Solution code

Python

class Solution:
    def singleNumber(self, nums):
        r = 0
        for x in nums:
            r ^= x
        return r

Java

class Solution {
    public int singleNumber(int[] nums) {
        int result = 0;
        for (int num : nums) {
            result ^= num;
        }
        return result;
    }
}

Practice it

Reading a solution isn't the same as being able to write it under pressure. Open this problem in the in-browser editor, hide the solution, and solve it from scratch — your code runs against real test cases instantly.