> ## Documentation Index
> Fetch the complete documentation index at: https://leetcode-py.wisl.dev/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> leetcode-py is a Python LeetCode practice environment generator with one CLI: lcpy. It is not a service or platform.
> Each problem is a directory under leetcode/ with README.md, solution.py, test_solution.py, helpers.py, and playground.ipynb. lcpy gen creates them from JSON templates bundled with the package.
> Examples are backed by tests; copy them verbatim.

# First Missing Positive Python Solution

> Tested Python solution for LeetCode 41 with 20 pytest cases. Generate a practice environment with lcpy.

LeetCode 41, Hard. Topics: Array, Hash Table. [View on LeetCode](https://leetcode.com/problems/first-missing-positive/description/).

Generate this problem as a practice environment: tested reference solution, 20 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 41   # by problem number
lcpy gen -s first_missing_positive   # by problem name
```

## Problem

Given an unsorted integer array `nums`. Return the smallest positive integer that is not present in `nums`.

You must implement an algorithm that runs in `O(n)` time and uses `O(1)` auxiliary space.

### Examples

```
Input: nums = [1,2,0]
Output: 3
Explanation: The numbers in the range [1,2] are all in the array.
```

```
Input: nums = [3,4,-1,1]
Output: 2
Explanation: 1 is in the array but 2 is missing.
```

```
Input: nums = [7,8,9,11,12]
Output: 1
Explanation: The smallest positive integer 1 is missing.
```

### Constraints

* `1 <= nums.length <= 10^5`
* `-2^31 <= nums[i] <= 2^31 - 1`

## Solution

Reference implementation from [solution.py on GitHub](https://github.com/wislertt/leetcode-py/blob/main/leetcode/first_missing_positive/solution.py), full suite in [test\_solution.py](https://github.com/wislertt/leetcode-py/blob/main/leetcode/first_missing_positive/test_solution.py):

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
class Solution:
    # Time: O(n)
    # Space: O(1)
    def first_missing_positive(self, nums: list[int]) -> int:
        n = len(nums)

        # Place each number in its correct position
        i = 0
        while i < n:
            correct_idx = nums[i] - 1
            if 1 <= nums[i] <= n and nums[i] != nums[correct_idx]:
                nums[i], nums[correct_idx] = nums[correct_idx], nums[i]
            else:
                i += 1

        # Find the first missing positive
        for i in range(n):
            if nums[i] != i + 1:
                return i + 1

        return n + 1
```

## Complexity

| Time | Space |
| ---- | ----- |
| O(n) | O(1)  |

## Tags

[Grind](/catalog/grind), [NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode), [AlgoMaster 75](/catalog/algo-master-75).