> For the complete documentation index, see [llms.txt](https://jaywin.gitbook.io/leetcode/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jaywin.gitbook.io/leetcode/solutions/0136-single-number.md).

# 0136. Single Number

<https://leetcode.com/problems/single-number>

## Description

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.

**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
```

**Constraints:**

* `1 <= nums.length <= 3 * 104`
* `-3 * 104 <= nums[i] <= 3 * 104`
* Each element in the array appears twice except for one element which appears only once.

## ac

```java
// b ^ b == 0, 0 ^ k == k, a ^ b ^ a ^ b ^ k == a ^ a ^ b ^ b ^ k == k
class Solution {
    public int singleNumber(int[] nums) {
        int res = 0;
        for (int i : nums) {
            res ^= i;
        }
        return res;
    }
}
```