请教一下，这段代码本来是求vector容量的，但是我输什么都直接编译return 0,咋改

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
class Comparator{
    public:
        Comparator(){diff=0;}
        virtual ~Comparator() {}
        virtual bool operator()(int a,int b)=0;
        int getDiff() {
            return diff;
        }
protected:
        int diff;
};
class GT :public Comparator{
public:
    bool operator()(int a,int b){
        diff=a-b;
        return a>b;
    }
};
class LT:public Comparator{
public:
    bool operator()(int a,int b){
        diff=b-a;
        return a<b;
    }
};
    int main(){
        char cmd= ' ';
        while(cmd != 'q'){
            cout<<"请输入排序命令，l为升序排序，g为降序排序，q为退出程序"<<endl;
            cin>>cmd;
            if(cmd='q'){
                break;
            }else if(cmd!='l'&&cmd!='g'){
                cout<<"不存在此命令，请重新输入："<<endl;
                continue;
            }
             vector<int>vec;
             cout<<"请输入vector的元素个数:"<<endl;
             int size=0;
             cin>>size;
             vec=vector<int>(size,0);
             cout<<"请输入vector的元素个数，一行一个："<<endl;
             for(int i=0;i<size;i++){
                 cin>>vec[i];
             }
             if(cmd=='l'){
                 sort(vec.begin(),vec.end(),LT());
             }else if(cmd=='g'){
                 sort(vec.begin(),vec.end(),GT());
             }
             cout<<"打印排序后的vector:"<<endl;
             for(int i=0;i<vec.size();i++){
                 cout<<vec[i]<<"";
             }
             cout<<endl;
        }
        cout<<"退出程序......"<<endl;
        return 0;
    }