題目：

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

[[2,4],[3,4],[2,3],[1,2],[1,3],[1,4], ]

思路：

遞歸

package recursion;import java.util.ArrayList; import java.util.List;public class Combinations {public List<List<Integer>> combine(int n, int k) {List<List<Integer>> res = new ArrayList<List<Integer>>();List<Integer> record = new ArrayList<Integer>();generateRecord(res, record, 1, n, k);return res;}private void generateRecord(List<List<Integer>> res, List<Integer> record, int start, int end, int k) {if (k == 0) {res.add(record);return;}for (int i = start; i <= end - k + 1; ++i) { List<Integer> newRecord = new ArrayList<Integer>(record);newRecord.add(i);generateRecord(res, newRecord, i + 1, end, k - 1);}}public static void main(String[] args) {// TODO Auto-generated method stubCombinations c = new Combinations();List<List<Integer>> res = c.combine(4, 2);for (List<Integer> l : res) {for (int i : l) System.out.print(i + "\t");System.out.println();}}}

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