LeetCode——77. 组合（深搜，回溯，剪枝）

题目描述：

参考代码：

 public List<List<Integer>> combine(int n, int k) {
    
    
        List<List<Integer>> lists=new ArrayList<>();
        dfs(lists,n,k,new ArrayList<Integer>(),1);
        return lists;
    }

    private void dfs(List<List<Integer>> lists, int n, int k,List<Integer> list,int start) {
    
    
        if(k==0){
    
    
            lists.add(new ArrayList<>(list));
            return;
        }
        for (int i = start; i <=n-k+1 ; i++) {
    
    
            list.add(i);
            dfs(lists,n,k-1,list,i+1);
            list.remove(list.size()-1);
        }

    }

  private List<List<Integer>> ans = new ArrayList<>();
    public List<List<Integer>> combine(int n, int k) {
    
    
        getCombine(n, k, 1, new ArrayList<>());
        return ans;
    }
    
    public void getCombine(int n, int k, int start, List<Integer> list) {
    
    
        if(k == 0) {
    
    
            ans.add(new ArrayList<>(list));
            return;
        }
        for(int i = start;i <= n - k + 1;i++) {
    
    
            list.add(i);
            getCombine(n, k - 1, i+1, list);
            list.remove(list.size() - 1);
        }
    }