> ## 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.

# Combination Sum II Python Solution with Tests

> Tested Python solution for LeetCode 40 with 14 pytest cases. Generate a practice environment with lcpy.

LeetCode 40, Medium. Topics: Array, Backtracking. [View on LeetCode](https://leetcode.com/problems/combination-sum-ii/description/).

Generate this problem as a practice environment: tested reference solution, 14 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 40   # by problem number
lcpy gen -s combination_sum_ii   # by problem name
```

## Problem

Given a collection of candidate numbers (`candidates`) and a target number (`target`), find all unique combinations in `candidates` where the candidate numbers sum to `target`.

Each number in `candidates` may only be used **once** in the combination.

**Note:** The solution set must not contain duplicate combinations.

### Examples

```
Input: candidates = [10,1,2,7,6,1,5], target = 8
Output: [[1,1,6],[1,2,5],[1,7],[2,6]]
```

```
Input: candidates = [2,5,2,1,2], target = 5
Output: [[1,2,2],[5]]
```

### Constraints

* 1 \<= candidates.length \<= 100
* 1 \<= candidates\[i] \<= 50
* 1 \<= target \<= 30

## Solution

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

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
class Solution:
    # Time: O(2^n)
    # Space: O(n)
    def combination_sum2(self, candidates: list[int], target: int) -> list[list[int]]:
        candidates.sort()
        result: list[list[int]] = []

        def backtrack(start: int, remaining: int, path: list[int]) -> None:
            if remaining == 0:
                result.append(path[:])
                return
            for i in range(start, len(candidates)):
                if i > start and candidates[i] == candidates[i - 1]:
                    continue
                if candidates[i] > remaining:
                    break
                path.append(candidates[i])
                backtrack(i + 1, remaining - candidates[i], path)
                path.pop()

        backtrack(0, target, [])
        return result
```

## Complexity

| Time   | Space |
| ------ | ----- |
| O(2^n) | O(n)  |

## Tags

[NeetCode 150](/catalog/neetcode-150), [NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode).