atoi函数位于cstring

#include<iostream>
#include<cstring>
using namespace std;
string s1;
char s2[20];
int main()
{
    cin>>s1;
    cin>>s2;
    int a=atoi(s1.c_str());
    int b=atoi(s2);
    cout<<"a="<<a<<endl;
    cout<<"b="<<b<<endl;
    return 0;
}