77.组合

给定两个整数 n 和 k，返回 1 ... n 中所有可能的 k 个数的组合。
示例:
输入: n = 4, k = 2
输出:
[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]

//回溯
class Solution {
  List> output = new LinkedList();
  int n;
  int k;

  public void backtrack(int first, LinkedList curr) {
    // if the combination is done
    if (curr.size() == k)
      output.add(new LinkedList(curr));

    for (int i = first; i < n + 1; ++i) {
      // add i into the current combination
      curr.add(i);
      // use next integers to complete the combination
      backtrack(i + 1, curr);
      // backtrack
      curr.removeLast();
    }
  }

  public List> combine(int n, int k) {
    this.n = n;
    this.k = k;
    backtrack(1, new LinkedList());
    return output;
  }

}