24点游戏的算法

判断是否可以算出24点，并输出一个可以算出24点的公式
import java.util.HashMap;
import java.util.Map;
import java.util.Stack;

/**
 * 1-13任取4个数，结果+-/*计算是否可以得到24，可以包括括号
 *@author javalixue
 */
public class TwentyFour {
    static Stack<Map<String, Object>> stack = new Stack<Map<String, Object>>();
    double[] arr = new double[4];

    public static void main(String[] args) {
        TwentyFour t = new TwentyFour();
        System.out.println(t.can24(6, 3, 8, 8));
        String formula = "";
        for (int i = stack.size() - 1; i >= 0; i--) {
            Map<String, Object> m = stack.get(i);
            String a = String.valueOf(m.get("a"));
            String b = String.valueOf(m.get("b"));
            Calculator c = (Calculator) m.get("calculator");
            if (i == stack.size() - 1) {
                formula = getCalculator(a, b, c);
            } else {
                formula = getCalculator(formula, b, c);
            }
        }
        System.out.println(formula);
    }

    static String getCalculator(String a, String b, Calculator c) {
        switch (c) {
        case Add:
            return "(" + a + "+" + b + ")";
        case Minus:
            return "(" + a + "-" + b + ")";
        case Multipy:
            return "(" + a + "*" + b + ")";
        case Divide:
            return "(" + a + "/" + b + ")";
        case AntiMinus:
            return "(" + b + "-" + a + ")";
        case AntiDivide:
            return "(" + b + "/" + a + ")";
        default:
            break;
        }
        return null;
    }

public boolean cal(int n) {

if (n == 1) {
            if (Math.abs(arr[0] - 24) < 1e-6) {
                return true;
            }
            return false;
        }

        for (int i = 0; i < n; i++) {
            for (int j = i + 1; j < n; j++) {
                double tmp1 = arr[i];
                double tmp2 = arr[j];
                arr[j] = arr[n - 1];// 用最后一位替换
                for (Calculator c : Calculator.values()) {
                    arr[i] = calculate(tmp1, tmp2, c);
                    if (cal(n - 1)) {
                        Map<String, Object> m = new HashMap<String, Object>();
                        m.put("a", (int) tmp1);
                        m.put("calculator", c);
                        m.put("b", (int) tmp2);
                        stack.add(m);
                        return true;
                    }
                }
                // 恢复数组
                arr[i] = tmp1;
                arr[j] = tmp2;
            }
        }
        return false;
    }

    public boolean can24(int a, int b, int c, int d) {
        this.arr = new double[] { a, b, c, d };
        return cal(4);

    }

    public double calculate(double a, double b, Calculator c) {
        switch (c) {
        case Add:
            return a + b;
        case Minus:
            return a - b;
        case Multipy:
            return a * b;
        case Divide:
            return a / b;
        case AntiMinus:
            return b - a;
        case AntiDivide:
            return b / a;
        default:
            return 0;
        }
    }
}

enum Calculator {
    Add, Minus, Multipy, Divide, AntiMinus, AntiDivide

}