Language: C++

#include<bits/stdc++.h>
//#include <ext/pb_ds/detail/standard_policies.hpp>
 
const double pi=acos(-1.0);
//memset ( a , 0 , n * sizeof(ll) ) ;
using namespace std;
//using namespace __gnu_pbds; 
#define   endl    '\n'
#define   sl(n)     scanf("%lld",&n)
#define   mp      make_pair
#define   pb      push_back
#define   ppb     pop_back
#define   fi      first
#define   se      second
#define   ll      long long
#define   ull     unsigned long long
#define   ld      long double
#define   pii     pair<int, int>
#define   f(i,a,b)  for(ll i = (ll)(a); i < (ll)(b); i++)
#define   rf(i,a,b)   for(ll i = (ll)(a); i > (ll)(b); i--)
#define   ms(a,b)   memset((a),(b),sizeof(a))
#define   abs(x)    ((x<0)?(-(x)):(x))
#define   MAX     1000005
#define   inf     LLONG_MAX
#define   ninf    LLONG_MIN
#define   MIN     INT_MIN
#define   yeet    return 0;
#define tihs if(fopen("input.txt","r"))freopen("input.txt", "r", stdin),freopen("output.txt", "w", stdout);

#define fast_io ios_base::sync_with_stdio (false) ; cin.tie(0) ; cout.tie(0) ;
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
// Use cout.flush() for interactive problems.
// Use this for increased stack size: g++ -o a.exe -Wl,--stack=256000000 -O2 source.cpp
inline long long  MAX2(long long  a, long long  b){return (a)>(b)?(a):(b);}
inline long long  MAX3(long long  a, long long  b,long long  c){return (a)>(b)?((a)>(c)?(a):(c)):((b)>(c)?(b):(c));}
inline long long  MIN2(long long  a, long long  b){return (a)<(b)?(a):(b);}
inline long long  MIN3(long long  a, long long b,long long c){return (a)<(b)?((a)<(c)?(a):(c)):((b)<(c)?(b):(c));}

//typedef
typedef long int int32;
typedef unsigned long int uint32;
typedef long long int int64;
typedef unsigned long long int  uint64;
 
 
int mod = 1e9 +7 ;
int64_t extGcd(int64_t a, int64_t b, int64_t& x, int64_t& y) {if (!a) {x = 0;y = 1;return b;}int64_t x1, y1;int64_t d = extGcd(b % a, a, x1, y1);x = y1 - (b / a) * x1;y = x1;return d;}
inline ll addmod(ll a,ll b){a=a%mod+b%mod;if(a>mod)a%=mod;return a;}
inline ll submod(ll a,ll b){a=a%mod-b%mod;if(a<0)a+=mod;return a;}
inline ll mulmod(ll a,ll b){return (a%mod * b%mod)%mod;}

int dx[]={1,1,0,-1,-1,-1, 0, 1};
int dy[]={0,1,1, 1, 0,-1,-1,-1}; 
inline ll exp(ll a,ll b){if(a==0)return 0ll;ll r=1LL;while(b>0){if(b&1){r=r*(a%mod);r=(r+mod)%mod;}b/=2;a=(a%mod)*(a%mod);a=(a+mod)%mod;}return (r+mod)%mod;}
ll gcd(ll a,ll b){if(b==0)return a;if(a==0)return b;return gcd(b,a%b);}
uint32 setbits(ll n){uint32 count=0;while (n){n&=(n-1);count++;}return count; } 

ll f[MAX];
ll iv[MAX];
ll C(ll n,ll r){
  if(n<r)return 0;
  ll ans=(f[n]%mod * iv[r]%mod)%mod;
  ans=(ans%mod * iv[n-r]%mod)%mod;
  return ans%mod;
}
void init(){
  f[0]=1;
     iv[0]=1;
     f(i,1,MAX)f[i]=(i*f[i-1])%mod;
     iv[MAX-1]=exp(f[MAX-1],mod-2);

     for(int i=MAX-2;i>=0;--i)iv[i]=(iv[i+1]*(i+1))%mod;
     
// }

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


 int main(){
     tihs;
     init();
      int n,m;
      cin>>n>>m;
    if(n+1 == m){
         ll ans= mulmod(f[n],f[m])
         cout<<ans<<endl;
    }
else if(m+1 ==n){
     ll ans= mulmod(f[n],f[m]);
     cout<<ans<<endl;
}
else if(n==m){
     ll ans= mulmod(2,mulmod(f[n],f[m]));
     cout<<ans<<endl;
}
else cout<<0<<endl;


     
      }

Compiler output:
a.cc: In function 'void init()':
a.cc:79:10: warning: empty parentheses were disambiguated as a function declaration [-Wvexing-parse]
   79 |  int main(){
      |          ^~
a.cc:79:10: note: remove parentheses to default-initialize a variable
   79 |  int main(){
      |          ^~
      |          --
a.cc:79:10: note: or replace parentheses with braces to value-initialize a variable
a.cc:79:12: error: a function-definition is not allowed here before '{' token
   79 |  int main(){
      |            ^
a.cc:100:8: error: expected '}' at end of input
  100 |       }
      |        ^
a.cc:66:12: note: to match this '{'
   66 | void init(){
      |            ^