# 137 Single Number II

Given an array of integers, every element appearsthreetimes except for one, which appears exactly once. Find that single one.

**Note:**\
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

**Complexity:** O(n) time and space

**The Idea:** Same as just finding a unique number. The problem is trivial (implemented below) if you don't take into account the fact that every number is referenced 3 times. The constant space solution likely involves some bitwise operation on all the elements.

```python
class Solution:
    def singleNumber(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """

        hash_count = {}

        for key in nums:
            if key in hash_count:
                hash_count[key] += 1
            else:
                hash_count[key] = 1

        for key, val in hash_count.items():
            if val == 1:
                return key


obj = Solution()
print(obj.singleNumber([1]))
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://maksimdan.gitbook.io/interview-practice-problems/leetcode_sessions/137-single-number-ii.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.