画解算法 77-组合

输入: n = 4, k = 2
输出:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

class Solution {
    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);
        }
    }
}