Modular Inverse 【欧几里得求 最小逆元】

The modular modular multiplicative inverse of an integer a modulo m is an integer x such that a-1≡x (mod m). This is equivalent to ax≡1 (mod m).

Input
There are multiple test cases. The first line of input is an integer T ≈ 2000 indicating the number of test cases.

Each test case contains two integers 0 < a ≤ 1000 and 0 < m ≤ 1000.

Output
For each test case, output the smallest positive x. If such x doesn’t exist, output “Not Exist”.

Sample Input
3
3 11
4 12
5 13
Sample Output
4
Not Exist
8

看代码吧

#include<stdio.h>#include<math.h>#include<iostream>using namespace std;const int MAXN =1E6;void exgcd(int a,int b,int &x,int &y,int &d){    if(b==0) { d=a; x=1;y=0;}    else {        exgcd(b,a%b,y,x,d);        y-=(a/b)*x;    }}int chi(int a,int n){    int x,y,d;    exgcd(a,n,x,y,d);    if(d!=1) return -1;    x=x%n;    if(x<=0) x+=n;      return x;}int main(){    int t;cin>>t;    while(t--){        int a,m;        cin>>a>>m;        int ans=chi(a,m);        if(ans==-1) puts("Not Exist");        else cout<<ans<<endl;    }}