题解 | #24点游戏算法#

import java.util.*;
import java.util.stream.*;
import java.io.*;
public class Main {
    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] strs = br.readLine().split(" ");
        int[] arr = new int[4];
        for (int i = 0; i < 4; i++) {
            arr[i] = Integer.parseInt(strs[i]);
        }
        // 数字有4x3x2x1=24种排列方式
        // 符号有4x4x4=64种排列方式
        System.out.println(calc(null, arr));

    }
    //计算res与剩余整数arr计算的值是否满足24点，若res为null表示为初次
    public static boolean calc(Double res, int[] arr) {
        if (arr.length == 0) {
            return res == 24;
        }
        for (int i = 0; i < arr.length; i++) {
            int cur = arr[i];
            int[] newList = getNewListRemoveI(arr, i);
            if (res == null) {
                if (calc(Double.valueOf(cur), newList)) {
                    return true;
                }
            } else if (calc(res + cur, newList) || calc(res - cur, newList) ||
                       calc(res * cur, newList) || calc(res / cur, newList)) {
                return true;
            }
        }
        return false;
    }

    //移除第i个值并创建新list
    public static int[] getNewListRemoveI(int[] arr, int removeIndex) {
        int[] newArr = new int[arr.length - 1];
        int j = 0;
        for (int i = 0; i < arr.length; i++) {
            if (i != removeIndex) {
                newArr[j++] = arr[i];
            }
        }
        return newArr;
    }
}