Java的全排列模板

c++有全排列函数，我们Java没有，所以我们只能自己手写了。

模板一：（不去重）

import java.util.ArrayList;
import java.util.Random;
import java.util.Scanner;
public class liyunpeng {public static void main(String[] args) {Scanner scan = new Scanner(System.in);int[] a = {1, 2, 3, 4};sum(a,0,a.length);}
public static void sum(int[] a, int start, int end ) {int b, c, d;if (start == end-1) {for (b = 0; b < end; b++) {System.out.print(a[b]);}System.out.println();return;}for (c = start; c < end; c++) {swap(a, c, start);sum( a, start +1, end);swap(a,c,start);}}
public static void swap ( int[] c, int a, int b){int d = c[a];c[a] = c[b];c[b] = d;}}

模板二：使用hashset来帮助我们去重

import java.util.*;public class liyunpeng {public static void main(String[] args) {Scanner scan = new Scanner(System.in);int[] a = {1, 9, 9, 4};sum(a,0,a.length);System.out.println(s.size());int f;int[] b = s.stream().mapToInt(Integer::intValue).toArray();for(int e=0;e<b.length;e++){for(f=e+1;f<b.length;f++){if(b[e]>b[f]){swap(b,e,f);}}}for(f=0;f<b.length;f++){System.out.println(b[f]);}}
public static HashSet<Integer> s=new HashSet<>();public static void sum(int[] a, int start, int end) {int b, c, d;int[] o=new int[50000];if (start == end-1) {int g=jisuan(a);if(sushu(g)==1){s.add(g);}return;}for (c = start; c < end; c++) {swap(a, c, start);sum( a, start +1, end);swap(a,c,start);}}
public static void swap ( int[] c, int a, int b){int d = c[a];c[a] = c[b];c[b] = d;}
public static int jisuan(int[] a){int b;b=a[0]*1000+a[1]*100+a[2]*10+a[3];return b;}
public static int sushu(int a){if(a<2||a==4)return 0;if(a==2||a==3)return 1;if(a%6!=5&&a%6!=1)return 0;for(int b=5;b*b<=a;b=b+6){if(a%b==0||a%(b+2)==0)return 0;}return 1;}}
*/