LeetCode Combinations

Given two integers n and k, return all possible combinations of k numbers out of 1,2,...,n.

import java.util.ArrayList;
import java.util.List;

class Solution {
    public List<List<Integer>> combine(int n, int k) {
        return combine(1, n + 1, k);
    }

    public List<List<Integer>> combine(int low, int upper, int k) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if (k == 1) {
            for (int i = low; i < upper; i++) {
                List<Integer> r = new ArrayList<Integer>();
                r.add(i);
                result.add(r);
            }
            return result;
        }
        for (int i = low; i < upper; i++) {
            List<List<Integer>> r = combine(i + 1, upper, k - 1);
            for (List<Integer> a : r) {
                a.add(0, i);
            }
            result.addAll(r);
        }
        return result;
    }

    public static void main(String[] args) {
        Solution s = new Solution();
        List<List<Integer>> lists = s.combine(4, 2);
        for(List res :lists){
            System.out.println(res);
        }
    }
};

 题目二：给定一个数组arr[]和一个整数k，在数组中选择元素组成一个长度为K的数组，打印最终的子数组

Example:arr={6，34，5，65}， K=2;

最终子数组：{6,34},{6,5},{6,65},{34,5},{34,65},{5,65}

参考代码:

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        int[] arr = {6, 34, 5, 65};
        int k = 5;
        // List<List<Integer>> lists = combine(1, 5, 2);
        List<List<Integer>> lists = combine2(arr, 0, arr.length, 2);
        for (List res : lists) {
            System.out.println(res);
        }
    }

    public static List<List<Integer>> combine2(int[] arr, int low, int upper, int k) {

        List<List<Integer>> res = new ArrayList<List<Integer>>();

        if (k == 1) {
            for (int i = low; i < upper; i++) {
                List<Integer> list = new ArrayList<>();
                list.add(arr[i]);
                res.add(list);
            }
            return res;
        }

        for (int i = low; i < upper; i++) {
            List<List<Integer>> r = combine2(arr, i + 1, upper, k - 1);
            for (List<Integer> l : r) {
                l.add(0, arr[i]);
            }
            res.addAll(r);
        }
        return res;
    }

}