# 0077. Combinations

<https://leetcode.com/problems/combinations>

## Description

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

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

**Example 1:**

```
**Input:** n = 4, k = 2
**Output:**
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]
```

**Example 2:**

```
**Input:** n = 1, k = 1
**Output:** [[1]]
```

**Constraints:**

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

## ac

```java
```
