public class Test {
    
    public static void main(String[] args) {
        int[] arr = new int[] { 1, 3, 5, 7, 9 };
        get(arr, 2);
    }

    public static void get(int[] a, int n) {
        if (n > a.length || n == 0) {
            return;
        }
        for (int i = n - 1; i < a.length; i++) {
            for (int j = 0; j < n - 1; j++) {
                System.out.print(a[j] + " ");
            }
            System.out.print(a[i] + "\n");
        }
        if (a.length - 1 < n || n == 1) {
            return;
        }
        int[] b = new int[a.length - 1];
        for (int i = 0; i < b.length; i++) {
            if (i == 0 && n != 2) {
                b[i] = a[i];
            } else {
                b[i] = a[i + 1];
            }
        }
        get(b, n);
    }
}

------解决方案--------------------
Java code

import java.util.Random;

public class Combination{
    
    int[] array={1,2,3};
    MyLinkedList newArray;
    boolean[] flags;
    int Cishu = 2;
    
    public Combination(int number){
        flags = new boolean[array.length];
        for(int i=0;i<array.length;i++){
            flags[i] = false;//表示该元素未取出
        }
        newArray = new MyLinkedList();
        this.Cishu = number;
    }
    public Combination(int[] array,int number){
        this.array = array;
        flags = new boolean[array.length];
        for(int i=0;i<array.length;i++){
            flags[i] = false;//表示该元素未取出
        }
        newArray = new MyLinkedList();
        this.Cishu = number;
    }
    
    public void getElement(){
        Random rd = new Random();
        int number = Math.abs(rd.nextInt()%array.length);
        while(flags[number]){//如果该数已经取出来了，丢弃，重取
            number = Math.abs(rd.nextInt()%array.length);
        }
        newArray.addElement(array[number]);
        flags[number] = true;
    }
    public void print(){
        for(int i=0;i<Cishu;i++){
            getElement();
        }
        Node node = newArray.head;
        while(node!=null){
            System.out.print(node.in+"|");
            node = node.next;
        }
    }
    public static void main(String args[]){
        
        new Combination(2).print();
        
    }
}
class MyLinkedList{
    Node head;
    Node tail;
    
    public void addElement(Integer in){
        if(head==null){
            Node node = new Node(in);
            head = node;
            tail = node;
            return;
        }
        if(head.in > in){
            Node node = new Node(in);
            head.prev = node;
            node.next = head;
            head = node;
            return;
        }
        Node zhizhen = head.next;
        while(zhizhen != null){
            if(zhizhen.in > in){//从小到大排序。
                Node node = new Node(in);
                zhizhen.prev.next = node;
                node.prev = zhizhen.prev;
                node.next = zhizhen;
                zhizhen.prev = node;
                return;
            }
            zhizhen = zhizhen.next;
        }
        Node node = new Node(in);//循环都没找到，说明是最大的
        tail.next = node;
        node.prev = tail;
        tail = node;
    }
}
class Node{//
    Integer in;
    Node next =null;//后向指针
    Node prev = null;//前向指针
    public Node(){
        
    }
    public Node(Integer in){
        this.in = in;
        this.next=null;
        this.prev=null;
    }
}

------解决方案--------------------
for example
Java code
public class Test {
    public static void main(String[] args) {
        combine(new int[]{1,2,3,4,5}, 3);
    }

    public static void combine(int[] a, int n) {
        if (n > a.length) {
            for (int i : a) {
                System.out.printf("%d ", i);
            }
            return;
        }

        int idx=0, t=0, p=(int)Math.pow(2, a.length);
        while (p > 0) {
            if (Integer.bitCount(p) == n) {
                t = p;
                idx = 0;
                while (t > 0) {
                    if (t%2==1) {
                        System.out.printf("%d ", a[idx]);
                    }
                    t >>= 1;
                    idx++;
                }
                System.out.println();
            }
            p--;
        }
    }
}