The Bits CodeForces - 1017B（組合數學，水）

注意要用long long

AC代碼：

#include <iostream>
#include <cstdio>
#include <string>
#include <cstring>
#include <algorithm>
using namespace std;
typedef long long ll;
const int maxn=1e6+5;
int main(){
	int n;
	cin>>n;
	string a,b;
	cin>>a>>b;
	ll num01=0,num10=0,num11=0,num00=0;
	for(int i=0;i<a.size();i++){
		if(a[i]=='0'){
			if(b[i]=='0')num00++;
			else num01++;
		}
		else if(a[i]=='1'){
			if(b[i]=='0')num10++;
			else num11++;
		}
	}
    //cout<<num0<<" "<<num1<<" "<<sum0<<" "<<sum1<<endl;
	cout<<num00*num10+num01*num10+num11*num00<<endl;
	return 0;
}