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=(<<);
 const double ESP=1e-;
 const int mod=(int)1e9+;
 const int N=(int)1e6+;

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

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

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

 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-);
 }

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

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

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