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package practice.recursion;
import java.util.ArrayList;
import java.util.List;
/**
* Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
* <p>
* You may return the answer in any order.
* <p>
*
* <p>
* Example 1:
* <p>
* Input: n = 4, k = 2
* Output:
* [
* [2,4],
* [3,4],
* [2,3],
* [1,2],
* [1,3],
* [1,4],
* ]
* Example 2:
* <p>
* Input: n = 1, k = 1
* Output: [[1]]
* <p>
* <p>
* 来源：力扣（LeetCode）
* 链接：https://leetcode-cn.com/problems/combinations
* 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。
*/
public class Combinations {
// 注意剪枝
public List<List<Integer>> combine(int n, int k) {
List<List<Integer>> resList = new ArrayList<>();
List<Integer> res = new ArrayList<>();
dfs(1, k, n, res, resList);
return resList;
}
public void dfs(int pos, int k, int n, List<Integer> res, List<List<Integer>> resList) {
if (res.size() == k) {
resList.add(new ArrayList<>(res));
return;
}
if (pos > n) return;
// drill down
res.add(pos);
dfs(pos + 1, k, n, res, resList); // remain
res.remove(res.size() - 1);
if (k - res.size() <= n - pos) // 剪枝
dfs(pos + 1, k, n, res, resList); // drop
}
}