USACO_2_3_Zero Sum

一个一个的穷举吧，在填充符号的时候按照‘ ’、‘+’、‘-’的顺序填充就是题目要求的所谓的“ASCII order”

/**//*
ID: sdjllyh1
PROG: zerosum
LANG: JAVA
complete date: 2008/12/18
complexity: O(3^n)
author: LiuYongHui From GuiZhou University Of China
more article: www.cnblogs.com/sdjls
*/

import java.io.*;
import java.util.*;

public class zerosum
{
    private static int n;
    private static char[] operators;//填充的符号
    private static ArrayList expression = new ArrayList();//符合要求的表达式

    public static void main(String[] args) throws IOException
    {
        init();
        run();
        output();
        System.exit(0);
    }

    private static void run()
    {
        insertOperator(1);
    }

    //递归填充符号，pos是填充位置
    private static void insertOperator(int pos)
    {
        //如果所有位置都填充完毕
        if (pos == n)
        {
            int sum = 0;//各个数字的和，即表达式的值
            int digit = 1;//当前遇到的数字
            int operator = 1;//当前遇到的数字前面的符号，-1表示减号
            for (int i = 1; i < n; i++)
            {
                switch (operators[i])
                {
                    case ' ':
                        digit = digit * 10 + i + 1;
                        break;
                    case '+':
                        sum += digit * operator;
                        operator = 1;
                        digit = i + 1;
                        break;
                    case '-':
                        sum += digit * operator;
                        operator = -1;
                        digit = i + 1;
                        break;
                    default:
                        String execution = "kill the USACO";
                        break;
                }
            }
            sum += digit * operator;

            if (sum == 0)
            {
                expression.add(makeAnExpression());
            }
        }
        else
        {
            //考虑三种情况，递归填充符号
            operators[pos] = ' ';
            insertOperator(pos + 1);

            operators[pos] = '+';
            insertOperator(pos + 1);

            operators[pos] = '-';
            insertOperator(pos + 1);
        }
    }

    private static String makeAnExpression()
    {
        StringBuffer exp = new StringBuffer();
        for (int i = 1; i < n; i++)
        {
            exp.append(i);
            exp.append(operators[i]);
        }
        exp.append(n);
        return exp.toString();
    }

    private static void init() throws IOException
    {
        BufferedReader f = new BufferedReader(new FileReader("zerosum.in"));
        n = Integer.parseInt(f.readLine());
        f.close();
        operators = new char[n];
    }

    private static void output() throws IOException
    {
        PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("zerosum.out")));
        Iterator it = expression.iterator();
        while (it.hasNext())
        {
            out.println(it.next());
        }
        out.close();
    }
}