检查所有可能等于给定4个整数的某个数字的表达式？

Question

import java.util.Scanner;

public class Game24 {

    public static void main(String[] args) {
        System.out.println("Enter 4 numbers and hit enter after each number: ");
        Scanner input = new Scanner(System.in);
        String num1 = input.nextLine();
        String num2 = input.nextLine();
        String num3 = input.nextLine();
        String num4 = input.nextLine();
        StringPermutation("", num1 + "+" + num2 + "*" + num3 + "/" + num4 + "(" + ")");
    }

    public static String StringPermutation(String x, String y) {
        double expression = 0;
        int count = 0;

        if (y.length() <= 1) {
            try {
                for (int i = 0; i < y.length(); i++) {
                    String p = y.substring(0, i) + y.substring(i + 1);
                    expression = Double.parseDouble(StringPermutation(x + y.charAt(i), p));
                }

                if (y.length() <= 1 && expression == 24) {
                    do {
                        count++;
                        System.out.println(x + y);
                    } while (count < 1);
                }

                //if (y.length() <= 1 && expression != 24) {
                //    System.out.print("No solution");
                //}
            } catch (Exception e) {}

        } else {
            for (int i = 0; i < y.length(); i++) {
                String p = y.substring(0, i) + y.substring(i + 1);
                StringPermutation(x + y.charAt(i), p);
            }
        }
        return "";
    }
}

Answer 1

以下是一些可能有助于您入门的设计提示：

>

当前转换为字符串然后测试排列的设计很差。它很难理解，而且容易出错。更好的方法是在计算表达式结果时构建表达式字符串。

生成解决方案列表的递归方法的一般形式如下：

List solutions=new ArrayList<>()；测试（partialSolution,startingContext）；

class Expressions {
    private final Set<String> validExpressions = new TreeSet<>();

    public Expressions(int expectedResult, int... operands) {
        test("" + operands[0], operands[0], Arrays.copyOfRange(operands, 1, operands.length);
    }

    private void test(String currentExpression, int currentResult, int... operands) {
        if (operands.length > 0) {
            ...
        } else if (currentResult == result) {
             validExpressions.add(expression + " = " + result);
        }
    }
}