> For the complete documentation index, see [llms.txt](https://garylai.gitbook.io/algorithm-and-data-structure/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://garylai.gitbook.io/algorithm-and-data-structure/problems/bit-manipulation/untitled.md).

# 136. Single Number

[136. Single Number](https://leetcode.com/problems/single-number/)

這題的想法是用消去法，很像是抽鬼牌，每次只要有成對的牌就消去，直到最後一張就是鬼牌了（答案）

```python
class Solution:
    def singleNumber(self, nums: List[int]) -> int:
        S = set()

        for num in nums:
            if num not in S:
                S.add(num)
            else:
                S.remove(num)

        return S.pop()
```

不過這題有一個很特別的解法，那就是靠位元操作去判斷。

```python
class Solution:
    def singleNumber(self, nums: List[int]) -> int:

        ans = 0
        for num in nums:
            ans ^= num

        return ans
```