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],
]

 本题也是采用回溯法，做法与之前的比较类似，代码如下：

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