去掉字符串中的多余空格

#include 
#include 

using namespace std;

string reverse(string s)
{	
	string res = "";
	string::size_type start = 0;
	string::size_type end = 0;
	int sflag = 0;
	int eflag = 0;
	for (int i = 0; i < s.size(); i++)
	{
		if (s[i] == ' ' && i + 1 < s.size() && isalnum(s[i + 1]))
		{
			start = i + 1;
			sflag = 1;
			
		}
		if (isalnum(s[i]) && i + 1 < s.size() && s[i + 1] == ' ')
		{
			end = i;
			eflag = 1;
		}
		if (sflag && eflag)
		{
			string tmp = s.substr(start, end - start + 1);
			//cout << tmp << endl;
			res = res + tmp + " ";
			sflag = 0;
			eflag = 0;
		}	
		
	}
	if (sflag == 1 && eflag == 0)
	{
		res += s.substr(start, s.size() - start);
	}
	cout << res << endl;
	return res;
}

int main()
{
	string s = "     mk     am      a student               ";
	string res = reverse(s);
	string t = "mk am a";
	cout << t << endl;
	system("pause");
	return 0;
}