P1009 [NOIP1998 普及组] 阶乘之和

  1. #include <bits/stdc++.h>
  3. using namespace std;
  5. const int N = 1e3 + 10;
  7. struct Bigint{
  8.     int len, a[N];
  10.     Bigint(int x = 0){
  11.         memset(a, 0, sizeof a);
  12.         len = 0;
  14.         if (x == 0) len = 1;
  15.         while (x){
  16.             a[len++] = x % 10;
  17.             x /= 10;
  18.         }
  19.     }
  21.     int &operator[](int i){
  22.         return a[i];
  23.     }
  25.     void flatten(int L){
  26.         len = L;
  27.         for (int i = 0; i < len; i++){
  28.             a[i + 1] += a[i] / 10;
  29.             a[i] %= 10;
  30.         }
  32.         while (a[len] == 0 && len > 0) len--;
  33.         len++;
  34.     }
  36.     void print(){
  37.         for (int i = len - 1; i >= 0; i--) printf("%d", a[i]);
  38.         puts("");
  39.     }
  40. };
  42. Bigint operator+(Bigint a, Bigint b){
  43.     Bigint c;
  44.     int len = max(a.len, b.len);
  45.     for (int i = 0; i < len; i++) c[i] += a[i] + b[i];
  46.     c.flatten(len + 1);
  47.     return c;
  48. }
  50. Bigint operator*(Bigint a, int b){
  51.     Bigint c;
  52.     int len = a.len;
  53.     for (int i = 0; i < len; i++) c[i] = a[i] * b;
  54.     c.flatten(len + 500);
  56.     return c;
  57. }
  59. int main(){
  60.     Bigint ans(0), fac(1);
  61.     int n;
  62.     cin >> n;
  63.     for (int i = 1; i <= n; i++){
  64.         fac = fac * i;
  65.         ans = ans + fac;
  66.     }
  68.     ans.print();
  70.     return 0;
  71. }
  1. 大整数运算，使用vector<>实现