# [77. Combinations](https://leetcode.com/problems/combinations/)


## 题目

Given two integers *n* and *k*, return all possible combinations of *k* numbers out of 1 ... *n*.

**Example**:

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

## 题目大意

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。

## 解题思路

- 计算排列组合中的组合，用 DFS 深搜即可，注意剪枝

## 代码

```go

package leetcode

func combine(n int, k int) [][]int {
	if n <= 0 || k <= 0 || k > n {
		return [][]int{}
	}
	c, res := []int{}, [][]int{}
	generateCombinations(n, k, 1, c, &res)
	return res
}

func generateCombinations(n, k, start int, c []int, res *[][]int) {
	if len(c) == k {
		b := make([]int, len(c))
		copy(b, c)
		*res = append(*res, b)
		return
	}
	// i will at most be n - (k - c.size()) + 1
	for i := start; i <= n-(k-len(c))+1; i++ {
		c = append(c, i)
		generateCombinations(n, k, i+1, c, res)
		c = c[:len(c)-1]
	}
	return
}

```