每日一题——组合

示例:
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>> result = new LinkedList();


    public List<List<Integer>> combine(int n, int k) {
        backtrack(n, k, 1, new LinkedList<Integer>());
        return result;
    }


    private void backtrack(int n, int k, int start, LinkedList<Integer> tmp) {
        if (tmp.size() == k) result.add(new LinkedList(tmp));
        for (int i = start; i <= n; i++) {
            tmp.add(i);
            backtrack(n, k, i + 1, tmp);
            tmp.removeLast();
        }
    }
    
}