中国剩余定理
一、中国剩余定理
对于k个两两互质的正整数:(m_1,m_2,...,m_k),任意k个正整数:(a_1, a_2, ..., a_k),一个未知数x,如果满足一下一次同余式组:
定义:
(M = m_1*m_2*...*m_k)
(M_i = M/m_i)
(M_i^{-1})表示(mod m_i)意义下的逆元(即:(M_i * M_i^{-1} ≡ 1(mod m_i)))
则其解为:
(x = a_1*M_1*M_1^{-1} + a_1*M_1*M_1^{-1} + ... + a_k*M_k*M_k^{-1})
代码:
#include <iostream>
using namespace std;
typedef long long LL;
const int N = 100;
LL exgcd(LL a, LL b, LL &x, LL &y)
{
if(!b)
{
x = 1, y = 0;
return a;
}
LL d = exgcd(b, a % b, y, x);
y -= (a / b) * x;
return d;
}
LL CRT(LL a[], LL m[], int n)
{
LL M = 1;
for(int i = 0; i < n; i++) M *= m[i];
LL res = 0;
for(int i = 0; i < n; i++)
{
LL x, y;
LL M_i = M / m[i];
LL d = exgcd(M_i, m[i], x, y);
res = (res + a[i] * M_i * x) % M;
}
return (res + M) % M;
}
int main()
{
LL a[N], m[N];
int n;
cin >> n;
for(int i = 0; i < n; i++)
scanf("%lld%lld", &a[i], &m[i]);
cout << CRT(a, m, n) << endl;
return 0;
}
二、利用拓展欧几里得求解一次同余式组
当(m_i)不满足两两互质时可以利用拓展欧几里得求解
推导过程:
对于两个同余式:
(left{ egin{aligned} x &≡ m_1(mod a_1) \ x &≡ m_2(mod a_2) \ end{aligned} ight.) 可得出:(left{ egin{aligned} x &= k_1 * a_1 + m_1 \ x &= k_2 * a_2 + m_2 \ end{aligned} ight.) 进一步可得出:(k_1 * a_1 - k_2 * a_2 = m_2 - m_1)
要使该方程有解则:(gcd(a_1, a_2) | m_2 - m_1)
令(d = gcd(a_1, a_2)),由拓展欧几里得定理可以求出方程的一个解为:(k_1 = k * (m_2 - m_1) / d)(其中,k为(k_1 * a_1 - k_2 * a_2 = gcd(k_1, k_2))的一个解),然后将其化为最小正整数解。
注:由于拓展欧几里得的通解为:(K = k + s * a_2/d(s为任意整数)),令(t = a_2/d),则最小正整数解为:((k\%t + t)\%t)
将方程中的(k_1、k_2)换元为:(left{ egin{aligned} k_1 &= k_1 + k * a_2 / d \ k_2 &= k_2 + k * a_1 / d\ end{aligned} ight.),带入原式可以得出:(x = (k_1 + k * a_2/d) * a_1 + m_1),展开得:(x = a_1*k_1 + m_1 + k*lcm(a_1, a_2))
令(a = lcm(a_1, a_2), m = a_1*k_1 + m_1),则:(x = k * a + m)
由此,两个同余式即化为了一个同余式,反复通过此方法,即可把n个同余式合并成1个,最终求出解。
代码:
#include <iostream>
#include <algorithm>
using namespace std;
typedef long long LL;
LL exgcd(LL a, LL b, LL &x, LL &y)
{
if(!b)
{
x = 1, y = 0;
return a;
}
LL d = exgcd(b, a % b, y, x);
y -= (a / b) * x;
return d;
}
int main()
{
int n;
cin >> n;
bool has_answer = true;
//作为第一组数据,同时存储每次合并后的结果
LL a1, m1;
cin >> a1 >> m1;
for(int i = 0; i < n-1; i++)
{
LL a2, m2;
scanf("%lld%lld", &a2, &m2);
LL k1, k2;
LL d = exgcd(a1, a2, k1, k2);
if((m2 - m1) % d)
{
has_answer = false;
break;
}
//求出一个解
k1 *= (m2 - m1) / d;
//求出最小整数解
LL t = a2 / d;
k1 = (k1 % t + t) % t;
//更新合并后的a1、m1
m1 = a1 * k1 + m1;
a1 = abs(a1 / d * a2);
}
//最小整数解
if(has_answer) cout << (m1 % a1 + a1) % a1 << endl;
else puts("-1");
return 0;
}
三、应用
广场上有一队士兵,如果排成10 列纵队,最后剩下a 个人((0 leq a leq 9));如果排成9 列纵队,最后剩下b 个人((0 leq b leq 8));如果排成8 列纵队,最后剩下c 个人((0 leq c leq 7))……如果排成2 列纵队,最后剩下i 个人((0 £ leq i leq 1)),输入a, b, c,…, i,输出广场上士兵的最少可能人数
设一共有x个士兵,由题意可以得出9个方程:(left{ egin{aligned} x \% 10 &= a_1 \ x \% 9 &= a_2 \ &...\ x \% 2 &= a_9end{aligned} ight.),则:(left{ egin{aligned} x &≡ a_1(mod 10) \ x &≡ a_2(mod 9) \ &......\ x &= a_9(mod 2) end{aligned} ight.),即题目转化为求一次同余式组。由于给出的模数并不是两两互质的,所以采用拓展欧几里得算法求解该问题。
代码:
#include <iostream>
using namespace std;
typedef long long LL;
LL exgcd(LL a, LL b, LL &x, LL &y)
{
if(!b)
{
x = 1, y = 0;
return a;
}
LL d = exgcd(b, a % b, y, x);
y -= (a / b) * x;
return d;
}
int main()
{
int n;
cin >> n;
int a[10];
cout << "请输入a~i:";
for(int i = 0; i < 9; i++)
scanf("%d", &a[i]);
bool has_answer = true;
//作为第一组数据,同时存储每次合并后的结果
LL a1 = 10, m1 = a[0];
for(int i = 0; i < n-1; i++)
{
LL a2 = 10 - i - 1, m2 = a[i + 1];
LL k1, k2;
LL d = exgcd(a1, a2, k1, k2);
if((m2 - m1) % d)
{
has_answer = false;
break;
}
//求出一个解
k1 *= (m2 - m1) / d;
//求出最小整数解
LL t = a2 / d;
k1 = (k1 % t + t) % t;
//更新合并后的a1、m1
m1 = a1 * k1 + m1;
a1 = abs(a1 / d * a2);
}
//最小整数解
if(has_answer) cout << (m1 % a1 + a1) % a1 << endl;
else puts("-1");
return 0;
}