POJ 1016 串

Time Limit Exceeded   why???

#include<iostream>
#include<algorithm> 
#include<sstream>
using namespace std;

string calInv(string s){
	int len=s.size();///>1
	string res;
	sort(&s[0],&s[len]);
	int count=1;
	for(int i=1;i<len;++i){///
		if(s[i]==s[i-1]){
			++count;
		}
		else{
			
			stringstream ss;
			string sc;
			ss<<count;
			ss>>sc;
			for(int j=0;j<sc.size();++j){
				res.push_back(sc[j]); 
			}
			res.push_back(s[i-1]);
			count=1;
		}
	}
	stringstream ss;
	string sc;
	ss<<count;
	ss>>sc;
	for(int j=0;j<sc.size();++j){
		res.push_back(sc[j]);
	} 
	res.push_back(s[len-1]);
	return res;
}

int main(){
	string s;
	while(cin>>s,s!="-1"){
		if(calInv(s)==s){
			cout<<s<<" is self-inventorying"<<endl;
			continue;
		}
		string tt[16];
		tt[0]=s;
		int it=0;
		for(;it<15;++it){
			//cout<<it<<endl;//
			tt[it+1]=calInv(tt[it]);
			//cout<<tt[it+1]<<endl;//
			if(tt[it+1]==tt[it]){
				cout<<s<<" is self-inventorying after " <<it<<" steps"<<endl;
				break;
			}
			int itt=0;
			for(;itt<it-1;++itt){
				if(tt[itt]==tt[it]){
					cout<<s<<" enters an inventory loop of length "<<it-itt<<endl;
					break;
				}
			}
			if(itt<it-1)
				break;
		}
		if(it==15)
			cout<<s<<" can not be classified after 15 iterations"<<endl;
	}
	return 0;
}

改了最后判断 enters an inventory loop of length 的部分 依然Time Limit Exceeded

                if(it==15){
			int itt=13;
			for(;itt>=0;--itt){
				if(tt[itt]==tt[it]){
					cout<<s<<" enters an inventory loop of length "<<it-itt<<endl;
					break;
				}
			}
			if(itt<0)
				cout<<s<<" can not be classified after 15 iterations"<<endl;
		}

请指正。