(dfs)解答110问题

在123456789之间插入+号或者-号，或者不加，使这个算术表达式的结果恰好是110

package JiaNan;
public class Test
{
	public static String operate[] = new String[]{"+","-"," "};
    public static void dfs(int m,int n,StringBuffer source)   //m是空间树层数
    {                                                         //n是待求和数字
		if(m == 17)                                             
		{
			if(sum(source) == n)
				System.out.println(source.toString().replace(" ","")+"=110");
			return;
		}
		else
		{
			for(int i = 0;i < 3;i++)
			{
				source.insert(m,operate[i]);
				dfs(m+2,n,source);
				source.delete(m,m+1);
			}
		}
    }
    public static int sum(StringBuffer source)
    {
		int sum = 0,start = 0;
		char oper =' ';
		String str = source.toString().replace(" ","");
		for(int i = 0;i <= str.length();i++)
		{
			if(i==str.length() || str.charAt(i)=='+' || str.charAt(i)=='-')
			{
				sum = Integer.parseInt(str.substring(start,i));
				if(i != str.length())
					oper = str.charAt(i);
				start = i+1;
				break;
			}
		}
		for(int i = start;i <= str.length();i++)
		{
			if(i==str.length() || str.charAt(i)=='+' || str.charAt(i)=='-')
			{
				if(oper == '+')
					sum += Integer.parseInt(str.substring(start,i));
				else
					sum -= Integer.parseInt(str.substring(start,i));
				start = i+1;
				if(i != str.length())
					oper = str.charAt(i);
			}
		}
		return sum;
		
    }
    public static void main(String args[])
    {
		StringBuffer source = new StringBuffer("123456789");
		dfs(1,110,source);
    }
}
/*
1+2+34+5+67-8+9=110
1+234-56-78+9=110
1-2+3+45-6+78-9=110
12+3+45+67-8-9=110
12+34+56+7-8+9=110
12-3+4-5+6+7+89=110
123+4+5+67-89=110
123+4-5-6-7-8+9=110
123-4+5-6-7+8-9=110
123-4-5+6+7-8-9=110
*/