走环概率问题（至今有点迷）--牛客第二场（ Eddy Walker）

思路：

概率结论题，好像属于线性递推，现在也不太懂（lll￢ω￢)

#define IOS ios_base::sync_with_stdio(0); cin.tie(0);
#include <cstdio>//sprintf islower isupper
#include <cstdlib>//malloc  exit strcat itoa system("cls")
#include <iostream>//pair
#include <fstream>
#include <bitset>
//#include <map>
//#include<unordered_map>
#include <vector>
#include <stack>
#include <set>
#include <string.h>//strstr substr
#include <string>
#include <time.h>//srand(((unsigned)time(NULL))); Seed n=rand()%10 - 0~9;
#include <cmath>
#include <deque>
#include <queue>//priority_queue<int, vector<int>, greater<int> > q;//less
#include <vector>//emplace_back
//#include <math.h>
//#include <windows.h>//reverse(a,a+len);// ~ ! ~ ! floor
#include <algorithm>//sort + unique : sz=unique(b+1,b+n+1)-(b+1);+nth_element(first, nth, last, compare)
using namespace std;//next_permutation(a+1,a+1+n);//prev_permutation
#define mem(a,b) memset(a,b,sizeof(a))
#define fo(a,b,c) for(a=b;a<=c;++a)//register int i
#define fr(a,b,c) for(a=b;a>=c;--a)
#define pr printf
#define sc scanf
void swapp(int &a,int &b);
double fabss(double a);
int maxx(int a,int b);
int minn(int a,int b);
int Del_bit_1(int n);
int lowbit(int n);
int abss(int a);
//const long long INF=(1LL<<60);
const double E=2.718281828;
const double PI=acos(-1.0);
const int inf=(1<<29);
const double ESP=1e-9;
const int mod=(int)1e9+7;
const int N=(int)1e6+6;

long long qpow(long long a,long long b,long long mod)
{
    long long ans;
    a%=mod;
    ans=1;
    while(b!=0)
    {
        if(b&1)
            ans=(ans*a)%mod;
        b/=2;
        a=(a*a)%mod;
    }
    return ans;
}

int main()
{
    int T;
    sc("%d",&T);
    long long ans=1,res;
    while(T--)
    {
        long long n,m;
        sc("%lld%lld",&n,&m);
        if(n==1)
        {
            res=1;
        }
        else
        {
            if(m==0)
            {
                res=0;
            }
            else
                res=qpow(n-1,mod-2,mod);
        }
        ans=ans*res;
        ans%=mod;
        pr("%lld
",ans);
    }
    return 0;
}

/**************************************************************************************/

int maxx(int a,int b)
{
    return a>b?a:b;
}

void swapp(int &a,int &b)
{
    a^=b^=a^=b;
}

int lowbit(int n)
{
    return n&(-n);
}

int Del_bit_1(int n)
{
    return n&(n-1);
}

int abss(int a)
{
    return a>0?a:-a;
}

double fabss(double a)
{
    return a>0?a:-a;
}

int minn(int a,int b)
{
    return a<b?a:b;
}