题解 | #24点游戏算法#

import java.util.Scanner;

public class Main {
    private static int isSolve = 0;
    public static void dfs(int index,int currentNum,int[] arr){
        if(currentNum == 24){
            isSolve = 1;
            return;// 结束递归
        }
        if(isSolve == 1 || currentNum > 24 || index >= 4)
        {
                return ;
        }
        for(int i = 0; i < 4;i++){
            switch (i){
                case 0:
                    dfs(index+1,currentNum - arr[index],arr);
                    break;
                case 1:
                    dfs(index+1,currentNum + arr[index],arr);
                    break;
                case 2:
                    dfs(index+1,currentNum * arr[index],arr);
                    break;
                case 3:
                    dfs(index+1,currentNum / arr[index],arr);
                    break;
            }
            if(isSolve == 1) return;
        }

    }
    public static void getAllOrder(int[] array,int start,int end){
        // 递归出口
        if(start == end){
            dfs(1,array[0],array);
            return;
        }else{
            for(int i = start;i < end;i++){
                getAllOrder(array,start + 1,end);
                swap(array,start,i);
            }
        }
    }
    // 交换两个元素的位置
    private static  void swap(int[] array,int i,int j){
        if(i == j){
            return;
        }
        int temp = temp = array[i];
        array[i] = array[j];
        array[j] = temp;
    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while(sc.hasNextInt()){
            int[] num = new int[4];
            for(int i = 0 ; i <= 3; i++)
                num[i] = sc.nextInt();
//            String temp = sc.nextLine();
            // 调用的数据要有一个全排列，这样才能保证全部情况都出现了，否则有可能漏掉
            getAllOrder(num,0,4);
            System.out.println(isSolve == 1 ? true : false +"");
//            System.out.println((flag == true ? true :false) +"");
        }

    }
}