# 组合
给定两个整数 n 和 k，返回 1 ... n 中所有可能的 k 个数的组合。
示例:
输入: n = 4, k = 2
输出:[[2,4],[3,4],[2,3],[1,2],[1,3],[1,4],]
## template
```java
import java.util.*;
public class Solution77 {
List> output = new LinkedList<>();
int n;
int k;
public void traceback(int first, LinkedList current) {
if (current.size() == k) {
output.add(new LinkedList(current));
System.out.println(output);
return;
}
for (int i = first; i <= n; i++) {
current.add(i);
traceback(i + 1, current);
current.removeLast();
}
}
public List> combine(int n, int k) {
this.n = n;
this.k = k;
traceback(1, new LinkedList<>());
return output;
}
}
```
## 答案
```java
```
## 选项
### A
```java
```
### B
```java
```
### C
```java
```