Language: C++

#include <bits/stdc++.h>
#define LLI long long int
#define FOR(v, a, b) for(LLI v = (a); v < (b); ++v)
#define FORE(v, a, b) for(LLI v = (a); v <= (b); ++v)
#define REP(v, n) FOR(v, 0, n)
#define REPE(v, n) FORE(v, 0, n)
#define REV(v, a, b) for(LLI v = (a); v >= (b); --v)
#define ALL(x) (x).begin(), (x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#define ITR(it, c) for(auto it = (c).begin(); it != (c).end(); ++it)
#define RITR(it, c) for(auto it = (c).rbegin(); it != (c).rend(); ++it)
#define EXIST(c,x) ((c).find(x) != (c).end())
#define fst first
#define snd second
#define popcount __builtin_popcount
#define UNIQ(v) (v).erase(unique(ALL(v)), (v).end())
#define bit(i) (1LL<<(i))

#ifdef DEBUG
#include <Mylib/Debug/debug.cpp>
#else
#define dump(...) ((void)0)
#endif

#define gcd __gcd

using namespace std;
template <class T> constexpr T lcm(T m, T n){return m/gcd(m,n)*n;}

template <typename I> void join(ostream &ost, I s, I t, string d=" "){for(auto i=s; i!=t; ++i){if(i!=s)ost<<d; ost<<*i;}ost<<endl;}
template <typename T> istream& operator>>(istream &is, vector<T> &v){for(auto &a : v) is >> a; return is;}
template <typename T> void puts_all(const T &value){std::cout << value << "\n";}
template <typename T, typename ...Args> void puts_all(const T &value, const Args&... args){std::cout << value << " ";puts_all(args...);}

template <typename T, typename U> bool chmin(T &a, const U &b){return (a>b ? a=b, true : false);}
template <typename T, typename U> bool chmax(T &a, const U &b){return (a<b ? a=b, true : false);}
template <typename T, size_t N, typename U> void fill_array(T (&a)[N], const U &v){fill((U*)a, (U*)(a+N), v);}
template <typename T> auto make_vector(int n, int m, const T &value){return vector<vector<T>>(n, vector<T>(m, value));}


struct Init{
  Init(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(12);
    cerr << fixed << setprecision(12);
  }
}init;

#include<experimental/optional>
namespace std{
template <typename T> using optional = std::experimental::optional<T>;
using nullopt_t = std::experimental::nullopt_t;
inline constexpr nullopt_t nullopt = std::experimental::nullopt;
}


template <std::uint32_t M> class ModInt{
public:
  std::uint64_t val;
  ModInt(): val(0){}
  ModInt(std::int64_t n){
    if(n >= M) val = n % M;
    else if(n < 0) val = n % M + M;
    else val = n;
  }
  
  inline constexpr ModInt operator+(const ModInt &a) const {return ModInt((val+a.val) % M);}
  inline constexpr ModInt operator-(const ModInt &a) const {return ModInt((val-a.val+M) % M);}
  inline constexpr ModInt operator*(const ModInt &a) const {return ModInt((val*a.val) % M);}
  inline constexpr ModInt operator/(const ModInt &a) const {return ModInt((val*a.inv().val) % M);}
  
  inline constexpr ModInt& operator=(const ModInt &a){val = a.val; return *this;}
  inline constexpr ModInt& operator+=(const ModInt &a){if((val += a.val) >= M) val -= M; return *this;}
  inline constexpr ModInt& operator-=(const ModInt &a){if(val < a.val) val += M; val -= a.val; return *this;}
  inline constexpr ModInt& operator*=(const ModInt &a){(val *= a.val) %= M; return *this;}
  inline constexpr ModInt& operator/=(const ModInt &a){(val *= a.inv().val) %= M; return *this;}

  inline constexpr bool operator==(const ModInt &a) const {return val == a.val;}
  inline constexpr bool operator!=(const ModInt &a) const {return val != a.val;}

  inline constexpr ModInt& operator++(){*this += 1; return *this;}
  inline constexpr ModInt& operator--(){*this -= 1; return *this;}

  inline constexpr ModInt operator++(int){ModInt t = *this; *this += 1; return t;}
  inline constexpr ModInt operator--(int){ModInt t = *this; *this -= 1; return t;}

  inline constexpr static ModInt frac(std::int64_t a, std::int64_t b){
    return ModInt(a) / ModInt(b);
  }
  
  inline constexpr static ModInt power(std::int64_t n, std::int64_t p){
    if(p < 0) return power(n, -p).inv();
    
    ModInt ret = 1, e = n;
    for(; p; e *= e, p >>= 1) if(p&1) ret *= e;
    return ret;
  }

  inline constexpr ModInt power(std::int64_t p) const {return power(val,p);}
  
  inline constexpr ModInt inv() const {
    std::int64_t a = val, b = M, u = 1, v = 0;
    
    while(b){
      std::int64_t t = a/b;
      a -= t*b; std::swap(a,b);
      u -= t*v; std::swap(u,v);
    }
    u %= M;
    if(u < 0) u += M;
    
    return u;
  }
};

template <std::uint32_t M> ModInt<M> operator-(const ModInt<M> &a){return M-a.val;}

template <std::uint32_t M> ModInt<M> operator+(std::int64_t a, const ModInt<M> &b){return ModInt<M>(ModInt<M>(a)+b.val);}
template <std::uint32_t M> ModInt<M> operator-(std::int64_t a, const ModInt<M> &b){return ModInt<M>(ModInt<M>(a)-b.val);}
template <std::uint32_t M> ModInt<M> operator*(std::int64_t a, const ModInt<M> &b){return ModInt<M>(ModInt<M>(a)*b.val);}
template <std::uint32_t M> ModInt<M> operator/(std::int64_t a, const ModInt<M> &b){return ModInt<M>(ModInt<M>(a)/b.val);}

template <std::uint32_t M> std::istream& operator>>(std::istream &is, ModInt<M> &a){is >> a.val; return is;}
template <std::uint32_t M> std::ostream& operator<<(std::ostream &os, const ModInt<M> &a){ os << a.val; return os;}


template <typename Cost = int> class Edge{
public:
  int from,to;
  Cost cost;
  Edge() {}
  Edge(int to, Cost cost): to(to), cost(cost){}
  Edge(int from, int to, Cost cost): from(from), to(to), cost(cost){}

  Edge rev() const {return Edge(to,from,cost);}
  
  friend std::ostream& operator<<(std::ostream &os, const Edge &e){
    os << "(FROM: " << e.from << "," << "TO: " << e.to << "," << "COST: " << e.cost << ")";
    return os;
  }
};

template <typename T> using Graph = std::vector<std::vector<Edge<T>>>;
template <typename T> using Tree = std::vector<std::vector<Edge<T>>>;

template <typename C, typename T> void add_edge(C &g, int from, int to, T w){
  g[from].push_back(Edge<T>(from, to, w));  
}

template <typename C, typename T> void add_undirected(C &g, int a, int b, T w){
  g[a].push_back(Edge<T>(a, b, w));
  g[b].push_back(Edge<T>(b, a, w));
}


template <typename T, typename U = std::int64_t>
class Dijkstra{
public:
  std::vector<std::optional<T>> dist;
  std::vector<U> path_count;

private:
  void run(const Graph<T> &graph, std::vector<int> src){
    const int n = graph.size();
    dist.assign(n, std::nullopt);
    path_count.assign(n, 0);
    
    std::vector<bool> check(n, false);
    std::priority_queue<std::pair<T,int>, std::vector<std::pair<T,int>>, std::greater<std::pair<T,int>>> pq;

    for(auto s : src){
      dist[s] = 0;
      pq.emplace(0, s);
      path_count[s] = 1;
    }

    while(not pq.empty()){
      const auto [d,i] = pq.top(); pq.pop();

      if(check[i]) continue;
      check[i] = true;

      for(auto &e : graph[i]){
        if(not dist[e.to]){
          dist[e.to] = d + e.cost;
          path_count[e.to] = path_count[e.from];
          pq.emplace(*dist[e.to], e.to);
        }else{
          if(*dist[e.to] > d + e.cost){
            dist[e.to] = d + e.cost;
            path_count[e.to] = path_count[e.from];
            if(not check[e.to]) pq.emplace(*dist[e.to], e.to);
          }else if(*dist[e.to] == d + e.cost){
            path_count[e.to] += path_count[e.from];
          }
        }
      }
    }
  }

public:
  Dijkstra(const Graph<T> &graph, int src){run(graph, {src});}
  Dijkstra(const Graph<T> &graph, const vector<int> &src){run(graph, src);}
};

const LLI mod = 1000000007;


using mint = ModInt<mod>;


int main(){
  int N, M, S, T;
  while(cin >> N >> M >> S >> T){
    --S, --T;

    Graph<LLI> g(N);
    REP(i,M){
      int u, v, d; cin >> u >> v >> d;
      --u, --v;
      add_undirected(g, u, v, (LLI)d);
    }


    auto res_s = Dijkstra<LLI, mint>(g, S);
    auto res_t = Dijkstra<LLI, mint>(g, T);


    auto ds = res_s.dist;
    auto dt = res_t.dist;
    
    auto count_from_s = res_s.path_count;
    auto count_from_t = res_t.path_count;
 
    LLI L = *ds[T];
 
    mint ans = count_from_s[T] * count_from_t[S];
 
    mint sub = 0;
 
    if(L % 2 == 0){
      REP(i,N){
        if(*ds[i] == L/2 and *dt[i] == L/2){
          mint temp = count_from_s[i] * count_from_t[i];
          sub += temp * temp;
        }
      }
    }
 
    for(auto &es : g){
      for(auto &e : es){
        if(*ds[e.from] + *dt[e.to] + e.cost == L and *ds[e.from] < L/2 and *dt[e.to] < L/2){
          mint temp = count_from_s[e.from] * count_from_t[e.to];
          sub += temp * temp;
        }
      }
    }
    
    ans -= sub;
 
    cout << ans << endl;
  }

  return 0;
}


Compiler output:
a.cc:52:29: error: conflicting declaration of template 'template<class T> using std::optional = std::experimental::fundamentals_v1::optional<_Tp>'
   52 | template <typename T> using optional = std::experimental::optional<T>;
      |                             ^~~~~~~~
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:79,
                 from a.cc:1:
/usr/include/c++/14/optional:68:11: note: previous declaration 'template<class _Tp> class std::optional'
   68 |     class optional;
      |           ^~~~~~~~
a.cc:53:7: error: conflicting declaration 'using std::nullopt_t = struct std::experimental::fundamentals_v1::nullopt_t'
   53 | using nullopt_t = std::experimental::nullopt_t;
      |       ^~~~~~~~~
/usr/include/c++/14/optional:71:10: note: previous declaration as 'struct std::nullopt_t'
   71 |   struct nullopt_t
      |          ^~~~~~~~~
a.cc:54:28: error: redefinition of 'constexpr const std::nullopt_t std::nullopt'
   54 | inline constexpr nullopt_t nullopt = std::experimental::nullopt;
      |                            ^~~~~~~
/usr/include/c++/14/optional:85:30: note: 'constexpr const std::nullopt_t std::nullopt' previously defined here
   85 |   inline constexpr nullopt_t nullopt { nullopt_t::_Construct::_Token };
      |                              ^~~~~~~