POJ - 2891 Strange Way to Express Integers——扩展欧几里得解同余方程

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
typedef long long ll;
const int maxn = 1e5 + 10;
ll a[maxn], r[maxn];
int n;
ll exgcd(ll a, ll b, ll& x, ll& y) {
    if (b == 0) {
        x = 1, y = 0;
        return a;
    }
    ll r = exgcd(b, a % b, y, x);
    y -= a/b * x;
    return r;
}
ll solve() {
    ll M = a[1], R = r[1], x, y;
    for (int i = 2; i <= n; i++) {
        ll d = exgcd(M, a[i], x, y);
        if ((R-r[i])%d != 0) return -1;
        x = (R-r[i])/d*x%(a[i]/d);
        R -= x*M;
        M = M / d * a[i];
        R %= M;
    }
    return (R % M + M) % M;
}
int main() {
    while (~scanf("%d", &n)) {
        for (int i = 1; i <= n; i++) scanf("%lld%lld", &a[i], &r[i]);
        printf("%lld\n", solve());
    }
    return 0;
}