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 ;
}
}
