# 77. Combinations

### Medium

---

Given two integers `n` and `k`, return *all possible combinations of* `k` *numbers chosen from the range* `[1, n]`.

You may return the answer in **any order**.

**Example 1:**

```plaintext
Input: n = 4, k = 2
Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
Explanation: There are 4 choose 2 = 6 total combinations.
Note that combinations are unordered, i.e., [1,2] and [2,1] are considered to be the same combination.
```

**Example 2:**

```plaintext
Input: n = 1, k = 1
Output: [[1]]
Explanation: There is 1 choose 1 = 1 total combination.
```

**Constraints:**

* `1 <= n <= 20`
    
* `1 <= k <= n`
    

## Solution:

```java
class Solution {
    private void f(int cur,int n,List<Integer> no,List<List<Integer>> ans,int k){
        if(no.size() == k){
            List<Integer> res = new ArrayList<>(no); 
            ans.add(res);
            return;
        }for(int i=cur;i<=n;i++) {
            no.add(i);
            f(i+1,n,no,ans,k);
            no.remove(no.size()-1);
        }
        
        
    }
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> ans = new ArrayList<>();
        List<Integer> no = new ArrayList<>();
        f(1,n,no,ans,k);
        return ans;
    }
}
```