求助 string怎么用啊

#include<iostream>
#include<cstdio>
#include<string>
using namespace std;
string a[7];
string b[22]={"zero","one","two","three","four","five",

"six","seven","eight","nine","ten","eleven", "twelve","thirteen","fourteen","fifteen",

"sixteen","seventeen","eighteen","nineteen","twenty"};

int c[7];
int main(){
    int num=0;
    for(int i=1;i<=6;i++){
        cin>>a[i];
        a[i][0]=tolower(a[i][0]);//第一个字符可能大写 转成小写
        for(int j=0;j<=20;j++){
            if(a[i]==b[j]){
                c[i]=j;
                num++;
                break;
            }
        }
    }
    //个位放前面少一位(开头为0去0) 一定小
    //剩下的都看作二位数(占两位) 个位数最高位为0 最小
    //总之从小到大排 
    int min=999;
    int p;
    for(int i=1;i<=num;i++){
        for(int j=1;j<=num;j++){
             if(min>c[j]){
                min=c[j];
                p=j;
            }
        }
        min=999;
        c[p]=999;
        //cout<<min;
        if(min==0)continue;
        else if(i!=1&&min<10)cout<<"0"<<min;
        else cout<<min;
    }
    return 0;
}