Combinations

题目地址：

https://leetcode.com/problems/combinations/#/description

题目：

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

For example,
If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

解题思路：

这道题就是dfs，然后控制一下list的size的大小。

代码：

public class Combinations {

    public List<List<Integer>> combine(int[] nums, int k) {
        List<List<Integer>> rst = new ArrayList<List<Integer>>();
        if(nums == null || nums.length == 0 || k == 0 || nums.length < k){
            return rst;
        }
        List<Integer> list = new ArrayList<Integer>();
        helper(nums, rst, list, k, 0);
        return rst;
    }

    private void helper(int[] nums, List<List<Integer>> rst, List<Integer> list, int k, int pos) {
        if(list.size() == k){
            rst.add(new ArrayList<>(list));
            return;
        }
        for(int i = pos; i <= nums.length - 1; i++){
            list.add(nums[i]);
            helper(nums, rst, list, k, i + 1);
            list.remove(list.size() - 1);
        }
    }

    public static void main(String[] args){
        int[] nums = {1, 2, 3, 4};
        Combinations combinations = new Combinations();
        List<List<Integer>> rst = combinations.combine(nums, 2);
        System.out.println(rst);
    }

}