我的代码错在了哪里？

Question

#include <bits/stdc++.h>
using namespace std;
int factorial(int n) {
    unsigned long long a = 1;
    for (int i = 1; i <= n; i++) {
        a *= i;
    }
    return a;
} 
int main() {
    unsigned long long m, n;
    cin >> m >> n;
    unsigned long long a1 = factorial(n);
    unsigned long long a2 = factorial(m) * factorial(n - m);
    cout << a1 / a2 << endl;
    return 0;
}

为啥输入19 20时输出168245652527而不是20？

Answer 1

因为你的factorial返回的是int,所以虽然fac里面用了ull,但是在返回的时候被截断了
将fac的返回值改为unsigned long long即可
代码修改如下,有帮助望采纳

 
#include <bits/stdc++.h>
using namespace std;
unsigned long long factorial(int n) {
    unsigned long long a = 1;
    for (int i = 1; i <= n; i++) {
        a *= i;
    }
    return a;
} 
int main() {
    unsigned long long m, n;
    cin >> m >> n;
    unsigned long long a1 = factorial(n);
    unsigned long long a2 = factorial(m) * factorial(n - m);
    cout << a1 / a2 << endl;
    return 0;
}