poj2891 Strange Way to Express Integers

扩展欧几里得，注意防溢出。

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

#include <cstdio>
using namespace std;
typedef __int64 LL;
const int maxn = 1e5 + 10;

LL a[maxn], r[maxn];
int n;
LL egcd(LL a, LL b, LL& x, LL& y){
    if(!b){
        x = 1, y = 0;
        return a;
    }
    LL d = egcd(b, a % b, x, y);
    LL t = x;
    x = y;
    y = t - a / b * y;
    return d;
}

void solve(){
    LL R = r[0], A = a[0], x, y;
    for(int i = 1; i < n; i++){
        LL a1 = A, b1 = a[i], m = r[i] - R;
        LL d = egcd(a1, b1, x, y);
        if(m % d != 0){
            printf("-1
");
            return;
        }
        LL t = A;
        LL B = a[i] / d;
        R += (m / d * x) % B * A;
        A *= B;
        if(R < 0) R += ((-R) / A + 2)* A;
        R %= A;
    }
    printf("%I64d
", R);
}

int main(){
    freopen("in.txt", "r", stdin);
    while(~scanf("%d", &n)){
        for(int i = 0; i < n; i++)    scanf("%I64d%I64d", &a[i], &r[i]);
        solve();
    }
    return 0;
}