77. 组合

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

    }
}