关于24点计算，我找到一个代码，大家分析一下他的思路？

Question

        private void button1_Click(object sender, EventArgs e)
        {
            Expression expression = new Expression();
            Thinker thinker = new Thinker(expression);
            List<int> card = new List<int>();
            int target = 24;
            if ((n1.Text== string.Empty)||( n2.Text== string.Empty )||( n3.Text== string.Empty) || (n4.Text==string.Empty)) {
                MessageBox.Show("请输入完整的四个数字！");
                return;
            }
            if ((int.Parse(n1.Text) < 0) ||( int.Parse(n1.Text) > 13)) {
                MessageBox.Show("第一个数溢出，重新输入！");
                n1.Focus();
                return;
            }
            if ((int.Parse(n2.Text) < 0) ||( int.Parse(n2.Text) > 13))
            {
                MessageBox.Show("第二个数溢出，重新输入！");
                n2.Focus();
                return;
            }
            if ((int.Parse(n3.Text) < 0) ||( int.Parse(n3.Text) > 13))
            {
                MessageBox.Show("第三个数溢出，重新输入！");
                n3.Focus();
                return;
            }
            if ((int.Parse(n4.Text) < 0) || (int.Parse(n4.Text) > 13))
            {
                MessageBox.Show("第四个数溢出，重新输入！");
                n4.Focus();
                return;
            }
            card.Add(int.Parse(this.n1.Text));
            card.Add(int.Parse(this.n2.Text));
            card.Add(int.Parse(this.n3.Text));
            card.Add(int.Parse(this.n4.Text));
            listBox1.Items.Clear();
            thinker.count(card, card.Count - 1, target, listBox1);
            if (listBox1.Items.Count==0)
            {
                MessageBox.Show("无法组成24点！！");
            }
        }

Answer 1

首先写两个类，第一个类是： Expression，
第二个类为：Thinker
代码分别如下：
Expression.cs
using System;
using System.Collections.Generic;
using System.Text;namespace _24点
{
    public class Expression
    {
        private List<int> _num = new List<int>();
        private List<String> _sign = new List<String>();
        public void add(int num)
        {
            _num.Add(num);
        }
        public void add(String sign)
        {
            _sign.Add(sign);
        }
        private int getPriority(String sign)
        {
            if (sign.Equals("+")) return 1;
            if (sign.Equals("-")) return 1;
            if (sign.Equals("*")) return 2;
            if (sign.Equals("/")) return 2;
            return -1;
        }
        private String toString(int layer)
        {
            if (layer == 0)
            {
                return _num[0] + _sign[0] + _num[1];
            }
            else
            {
                String result = this.toString(layer - 1);
                if (getPriority(_sign[layer]) > getPriority(_sign[layer - 1]))
                    result = "(" + result + ")";
                result += _sign[layer] + _num[layer + 1];
                return result;
            }
        }
        public String toString()
        {
            return toString(2);
        }
        public void clear()
        {
            _num.Clear();
            _sign.Clear();
        }
    }
}

Answer 2

Thinker.cs
using System;
using System.Collections.Generic;
using System.Text;
using System.Collections;//add
using System.Windows.Forms;
namespace _24点
{
    public class Thinker
    {
        private Expression _expression;
        public Thinker(Expression expression)
        {
            _expression = expression;
        }
        public bool count(List<int> array, int num, int target, ListBox box)
        {
            if (num == 1)
            {
                if ((array[0] + array[1]) == target)
                {
                    _expression.add(array[0]);
                    _expression.add(array[1]);
                    _expression.add("+");
                    return true;
                }
                if ((array[0] - array[1]) == target)
                {
                    _expression.add(array[0]);
                    _expression.add(array[1]);
                    _expression.add("-");
                    return true;
                }
                if ((array[1] - array[0]) == target)
                {
                    _expression.add(array[1]);
                    _expression.add(array[0]);
                    _expression.add("-");
                    return true;
                }
                if ((array[0] * array[1]) == target)
                {
                    _expression.add(array[0]);
                    _expression.add(array[1]);
                    _expression.add("*");
                    return true;
                }
                if ((array[0] * target == array[1]))
                {
                    _expression.add(array[1]);
                    _expression.add(array[0]);
                    _expression.add("/");
                    return true;
                }
                if ((array[1] * target == array[0]))
                {
                    _expression.add(array[0]);
                    _expression.add(array[1]);
                    _expression.add("/");
                    return true;
                }
                return false;
            }
            else
            {
                for (int current = 0; current < array.Count; current++)
                {
                    List<int> array1 = new List<int>();
                    int currentNum = array[current];
                    for (int i = 0; i < array.Count; i++)
                        if (i != current) array1.Add(array[i]); //array
                    if (count(array1, num - 1, target - currentNum, box))
                    {
                        _expression.add("+");
                        _expression.add(currentNum);
                        if (num == 3)
                        {
                            box.Items.Add(_expression.toString());
                            //MessageBox.Show(_expression.toString());
                            _expression.clear();
                        }
                        if (num != 3) return true;
                    }
                    if (count(array1, num - 1, target + currentNum, box))
                    {
                        _expression.add("-");
                        _expression.add(currentNum);
                        if (num == 3)
                        {
                            box.Items.Add(_expression.toString());
                            _expression.clear();
                        }
                        if (num != 3) return true;
                    }
                    if (count(array1, num - 1, target * currentNum, box))
                    {
                        _expression.add("/");
                        _expression.add(currentNum);
                        if (num == 3)
                        {
                            box.Items.Add(_expression.toString());
                            _expression.clear();
                        }
                        if (num != 3) return true;
                    }
                    if (target % currentNum == 0)
                    {
                        if (count(array1, num - 1, (int)(target / currentNum), box))
                        {
                            _expression.add("*");
                            _expression.add(currentNum);
                            if (num == 3)
                            {
                                box.Items.Add(_expression.toString());
                                _expression.clear();
                            }
                            if (num != 3) return true;
                        }
                    }
                }
                return false;
            }
        }
    }
}

Answer 3

思路,算法在这两行代码里
Expression expression = new Expression(); 
Thinker thinker = new Thinker(expression); 
把这些代码贴出来，看看

Answer 4

运行效果可以将结果表达式放到列表框中了。
if (num != 3) return true;可否去掉？
我去掉就不对了？