洛谷 同余方程

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#include<bits/stdc++.h>
using namespace std;
long long exgcd(long long a, long long b, long long &x1, long long &y1){
    if (b == 0){
        x1 = 1; y1 = 0;
        return a;
    }
    long long x, y;
    long long r = exgcd(b, a%b, x, y);
    x1 = y;
    y1 = x - a / b*y;
    return r;
}
int main(){
    ios::sync_with_stdio(false);
    long long a, b;
    cin >> a >> b;
    long long x, y;
    exgcd(a, b, x, y);
    cout << (x + b) % b;

}

做题做的心力憔悴
20180807更新:
exgcd证明过程参见:https://renjikai.com/cpp-number-theory/

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#include<iostream>
#include<algorithm>
#include<vector>
#include<queue>
#include<cstring>
#define ll long long
#define pii pair<int,int>
#define PINF 0x7fffffff
#define NINF 0x80000000
using namespace std;
ll exgcd(ll a, ll b, ll &x, ll &y) {
    if (b == 0) {
        x = 1; y = 0;
        return a;
    }
    ll d = exgcd(b, a%b, y, x);
    y -= a / b * x;
    return d;
}
int main() {
    ll a, b, x, y;
    cin >> a >> b;
    exgcd(a, b, x, y);
    cout << (x%b + b) % b;
}

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