样例
#include <iostream>
using namespace std;
//判断0-n出现2的次数
int cnt_2(int n){
int c = 1, res = 0, rem = 0;
while(n){
if(n % 10 == 2) res += n / 10 * c + rem + 1;
else res += (n / 10 + (n % 10 > 1)) * c;
rem += n % 10 * c;
c *= 10;
n /= 10;
}
return res;
}
int main(){
int l, r, res = 0, n = 1;
cin >> l >> r;
cout << cnt_2(r) - cnt_2(l - 1);
}