LeetCode题目9 Count and Say

题目：

The count-and-say sequence is the sequence of integers beginning as follows:
1, 11, 21, 1211, 111221, ...

1 is read off as "one 1" or 11.
11 is read off as "two 1s" or 21.
21 is read off as "one 2, then one 1" or 1211.

Given an integer n, generate the n^th sequence.

Note: The sequence of integers will be represented as a string. 

代码：

string cas_next(const string & cas) {

	string ret;
	char cur_str=cas.at(0);
	int num=0;
	for (int i=0;i<cas.length();i++)
	{
			if (cur_str==cas.at(i))
			{
				num++;
			}
			else{
				stringstream ss;
				ss<<num<<cur_str;
				ret+=ss.str();
				cur_str=cas.at(i);
				num=1;
			}
	}
	 if (num > 0)     {      
		 stringstream ss;                   
		 ss << num << cur_str;                
		 ret += ss.str();      
	 }
	return ret;
	
}

string countAndSay(int n) {
        
     if (n < 1)     return ""; 
     string s="1";
    for (int i=1;i<n;i++)
   {
       s=cas_next(s);
   }
    return s;
}