# 0040. Combination Sum II

<https://leetcode.com/problems/combination-sum-ii>

## Description

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.

**Example 1:**

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

**Example 2:**

```
**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`

## ac

```java
class Solution {
    public List<List<Integer>> combinationSum2(int[] candidates, int target) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        Arrays.sort(candidates);
        backtrack(candidates, 0, target, res, new ArrayList<Integer>());
        return res;
    }

    private void backtrack(int[] candidates, int start, int remain, List<List<Integer>> res, List<Integer> note) {
        // exit
        if (remain < 0) return;
        if (remain == 0) {
            res.add(new ArrayList<Integer>(note));
            return;
        }

        for (int i = start; i < candidates.length; i++) {
            if (i > start && candidates[i] == candidates[i-1]) continue; // skip duplicates, only take start point value
            note.add(candidates[i]);
            backtrack(candidates, i+1, remain-candidates[i], res, note);
            note.remove(note.size()-1);
        }
    }
}
```


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