poj2891 Strange Way to Express Integers

扩展欧几里得，注意防溢出。

http://poj.org/problem?id=2891

 1 #include <cstdio>
 2 using namespace std;
 3 typedef __int64 LL;
 4 const int maxn = 1e5 + 10;
 5
 6 LL a[maxn], r[maxn];
 7 int n;
 8 LL egcd(LL a, LL b, LL& x, LL& y){
 9     if(!b){
10         x = 1, y = 0;
11         return a;
12     }
13     LL d = egcd(b, a % b, x, y);
14     LL t = x;
15     x = y;
16     y = t - a / b * y;
17     return d;
18 }
19
20 void solve(){
21     LL R = r[0], A = a[0], x, y;
22     for(int i = 1; i < n; i++){
23         LL a1 = A, b1 = a[i], m = r[i] - R;
24         LL d = egcd(a1, b1, x, y);
25         if(m % d != 0){
26             printf("-1\n");
27             return;
28         }
29         LL t = A;
30         LL B = a[i] / d;
31         R += (m / d * x) % B * A;
32         A *= B;
33         if(R < 0) R += ((-R) / A + 2)* A;
34         R %= A;
35     }
36     printf("%I64d\n", R);
37 }
38
39 int main(){
40     freopen("in.txt", "r", stdin);
41     while(~scanf("%d", &n)){
42         for(int i = 0; i < n; i++)    scanf("%I64d%I64d", &a[i], &r[i]);
43         solve();
44     }
45     return 0;
46 }