LeetCode

题目：

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：

递归

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 stub
        Combinations c = new Combinations();
        List<List<Integer>> res = c.combine(4, 2);
        for (List<Integer> l : res) {
            for (int i : l) 
                System.out.print(i + "	");
            System.out.println();
        }
    }

}