LeetCode 77. Combinations

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],]

---

又是一道全排列问题，这种题的处理方法就是用一个list来进行递归处理。

public class Solution {    public List<List<Integer>> combine(int n, int k) {        List<List<Integer>>res = new ArrayList<List<Integer>>();        if(k==0||k>n)return res;        helper(res,n,k,1,new ArrayList<Integer>());        return res;    }    public void helper(List<List<Integer>>res,int n,int k,int j,List<Integer>list){        if(list.size()==k){            res.add(new ArrayList<Integer>(list));            return ;        }        for(int i=j;i<=n;i++){            //list = new ArrayList<Integer>();            list.add(i);                        helper(res,n,k,i+1,list);            list.remove(list.size()-1);        }        return ;    }}