【JAVA】24点 递归穷举

import java.util.ArrayList;public class JavaTest {/** * 使用4张牌计算出24点，只使用加减乘除四种运算符 * @param cards : 输入的4张牌 * @return 计算24点的算式 */public static String res=null;public String make24Points(String cards) {if(cards==null||cards.contains("joker")||cards.contains("JOKER")){return "ERROR";}String[] c=cards.split(" ");if(c.length!=4){return "ERROR";}ArrayList<Double> d=new ArrayList<Double>();ArrayList<String> s=new ArrayList<String>();for(int i=0;i<4;i++){double td=toD(c[i]);if(td==-1){return "ERROR";}d.add(td);s.add(c[i]);}res=null;//清空数据recursion(d,s);if(res==null){return "NONE";}return res;}//递归，数组中取出两个数计算后结果放回数组private void recursion(ArrayList<Double> d,ArrayList<String> s){int n=d.size();if(n==1){if(Math.abs(d.get(0)-24.0)<=0.000000001){//防止有分数而产生误差res=s.get(0);//若数组只有一个数，且为24，则将算式字符串赋值给类静态变量//System.out.println(res);}return;}for(int i=0;i<n;i++){for(int j=i+1;j<n;j++){ArrayList<Double> dd=new ArrayList<Double>();//用于递归的新数组ArrayList<String> ss=new ArrayList<String>();//用于存放算式的新数组//任意取出两个数的不同组合情况for(int r=0;r<n;r++){if(r!=i&&r!=j){dd.add(d.get(r));ss.add(s.get(r));}}dd.add(d.get(i)+d.get(j));//加入取出的两数计算结果 a+bss.add("("+s.get(i)+"+"+s.get(j)+")");//记录两数计算的算式recursion(dd, ss);//递归if(res!=null){//若静态变量已有值，则返回return;}dd.remove(dd.size()-1);//移除上次添加的两数计算结果ss.remove(ss.size()-1);dd.add(d.get(i)-d.get(j));//a-bss.add("("+s.get(i)+"-"+s.get(j)+")");recursion(dd, ss);if(res!=null){return;}dd.remove(dd.size()-1);ss.remove(ss.size()-1);dd.add(d.get(i)*d.get(j));//a*bss.add(s.get(i)+"*"+s.get(j));recursion(dd, ss);if(res!=null){return;}dd.remove(dd.size()-1);ss.remove(ss.size()-1);dd.add(d.get(i)/d.get(j));//a/bss.add(s.get(i)+"/"+s.get(j));recursion(dd, ss);if(res!=null){return;}dd.remove(dd.size()-1);ss.remove(ss.size()-1);dd.add(d.get(j)-d.get(i));//b-ass.add("("+s.get(j)+"-"+s.get(i)+")");recursion(dd, ss);if(res!=null){return;}dd.remove(dd.size()-1);ss.remove(ss.size()-1);dd.add(d.get(j)/d.get(i));//b/ass.add(s.get(j)+"/"+s.get(i));recursion(dd, ss);if(res!=null){return;}}}}//转换牌为数字private double toD(String s){if(s.equals("A")){return 1;}if(s.equals("J")){return 11;}if(s.equals("Q")){return 12;}if(s.equals("K")){return 13;}int n=Integer.parseInt(s);if(n<2||n>10){return -1;}return n;}public static void main(String[] args) {JavaTest c = new JavaTest();String cards = "Q A 6 A";//String cards = "5 5 5 A";System.out.println("The method to make 24 points is: " + c.make24Points(cards));}}

界面（可忽略）：

import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import javax.swing.*;public class View {JFrame frame=new JFrame("24点小游戏");JPanel mainPanel=new JPanel();JTextField pork=new JTextField();JLabel res=new JLabel();JButton calButton=new JButton("结果");JButton reButton=new JButton("发牌");public void go(){mainPanel.setLayout(null);mainPanel.add(pork);mainPanel.add(reButton);mainPanel.add(res);mainPanel.add(calButton);pork.setBounds(20,18,150,30);reButton.setBounds(190,18,78,30);res.setBounds(20,66,150,30);calButton.setBounds(190,66,78,30);reButton.addActionListener(new ReButtonListener());calButton.addActionListener(new CalButtonListener());frame.add(mainPanel);frame.setSize(300, 150);frame.setLocationRelativeTo(null);frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  frame.setVisible(true);}class ReButtonListener implements ActionListener{@Overridepublic void actionPerformed(ActionEvent arg0) {pork.setText(getRadom()+" "+getRadom()+" "+getRadom()+" "+getRadom());res.setText("");}}class CalButtonListener implements ActionListener{@Overridepublic void actionPerformed(ActionEvent arg0) {String s=pork.getText();res.setText(new JavaTest().make24Points(s));res.repaint();}}private static String getRadom(){String[] pork ={"A","2","3","4","5","6","7","8","9","10","J","Q","K"};return pork[(int)(Math.random()*13)];}public static void main(String[] args) {new View().go();}}