Bài 10. Viết chương trình liệt kê tất cả các tập con k phần tử của 1, 2, ..,n (k≤n).

import java.util.Scanner;

public class Main {

    public static int nhap(){

                        Scanner input= new Scanner(System.in);

                        boolean check= false;

                        int n=0;

                        while(!check){

                                    System.out.print(" ");

                                    try{

                                                n= input.nextInt();

                                                check= true;

                                    }catch(Exception e){

                                                System.out.println("Ban phai nhap so! hay nhap lai...");

                                                input.nextLine();

                                    }

                        }

                        return (n);

            }

    public static void result(int a[],int k){

                        int i;

                        System.out.println();

                        for(i=1 ; i<=k ; i++){

                                    System.out.print("  "+a[i]);

                        }

            }

            public static void try_backTrack(int a[], int n, int k, int i){

                        int j;

                        for(j=a[i-1]+1 ; j<=(n-k+i) ; j++){

                                    a[i]= j;

                                    if(i== k) result(a,k);

                                    else try_backTrack(a, n, k, i+1);

                        }

            }

    public static void main(String[] args) {

        System.out.println("Nhap n");

                        int n= nhap();

                        int[] array= new int[n+1];

                        int k;

                        System.out.println("Liet ke tat ca cac tap con k phan tu cua 1,2,..,"+n+" : ");

                        for(k=1 ; k<=n ; k++){

                                    System.out.println("\n Tap con "+k+" phan tu: ");

                                    try_backTrack(array,n,k,1);

                        }

    }

}