s取一个得到char，char转int

不需要函数，直接-‘0’

#include<bits/stdc++.h>

using namespace std;
int main()
{
    string s = "1234";
    char t = s[1];
    int i = t;
    cout<<i-'0';
    return 0;
}

substr和string转int

#include<iostream>

using namespace std;
int main()
{
    string s = "1234";
    string start = s.substr(0,1);
    string end = s.substr(s.size()-1,1);
    int a = stoi(start);
    int b = stoi(end);
    cout<<start<< " " << end<<endl;
    cout<<a<< " " << b;
    return 0;
}