#include #pragma GCC diagnostic ignored "-Wsign-compare" #pragma GCC diagnostic ignored "-Wsign-conversion" #define NDEBUG #define SHOW(...) static_cast(0) //!===========================================================!// //! dP dP dP !// //! 88 88 88 !// //! 88aaaaa88a .d8888b. .d8888b. .d888b88 .d8888b. 88d888b. !// //! 88 88 88ooood8 88' '88 88' '88 88ooood8 88' '88 !// //! 88 88 88. ... 88. .88 88. .88 88. ... 88 !// //! dP dP '88888P' '88888P8 '88888P8 '88888P' dP !// //!===========================================================!// using ld = long double; using uint = unsigned int; using ll = long long; using ull = unsigned long long; constexpr unsigned int MOD = 1000000007; template constexpr T INF = std::numeric_limits::max() / 4; template constexpr F PI = static_cast(3.1415926535897932385); std::mt19937 mt{std::random_device{}()}; template bool chmin(T& a, const T& b) { return a = std::min(a, b), a == b; } template bool chmax(T& a, const T& b) { return a = std::max(a, b), a == b; } template std::vector Vec(const std::size_t n, T v) { return std::vector(n, v); } template auto Vec(const std::size_t n, Args... args) { return std::vector(n, Vec(args...)); } template constexpr T popCount(const T u) { #ifdef __has_builtin return u == 0 ? T(0) : (T)__builtin_popcountll(u); #else unsigned long long v = static_cast(u); return v = (v & 0x5555555555555555ULL) + (v >> 1 & 0x5555555555555555ULL), v = (v & 0x3333333333333333ULL) + (v >> 2 & 0x3333333333333333ULL), v = (v + (v >> 4)) & 0x0F0F0F0F0F0F0F0FULL, static_cast(v * 0x0101010101010101ULL >> 56 & 0x7f); #endif } template constexpr T log2p1(const T u) { #ifdef __has_builtin return u == 0 ? T(0) : T(64 - __builtin_clzll(u)); #else unsigned long long v = static_cast(u); return v = static_cast(v), v |= (v >> 1), v |= (v >> 2), v |= (v >> 4), v |= (v >> 8), v |= (v >> 16), v |= (v >> 32), popCount(v); #endif } template constexpr T clog(const T v) { return v == 0 ? T(0) : log2p1(v - 1); } template constexpr T msbp1(const T v) { return log2p1(v); } template constexpr T lsbp1(const T v) { #ifdef __has_builtin return __builtin_ffsll(v); #else return v == 0 ? T(0) : popCount((v & (-v)) - T(1)) + T(1); #endif } template constexpr bool ispow2(const T v) { return popCount(v) == 1; } template constexpr T ceil2(const T v) { return v == 0 ? T(1) : T(1) << log2p1(v - 1); } template constexpr T floor2(const T v) { return v == 0 ? T(0) : T(1) << (log2p1(v) - 1); } //!===============================================================!// //! 88888888b dP .88888. a88888b. 888888ba !// //! 88 88 d8' '88 d8' '88 88 '8b !// //! a88aaaa dP. .dP d8888P 88 88 88 88 !// //! 88 '8bd8' 88 88 YP88 88 88 88 !// //! 88 .d88b. 88 Y8. .88 Y8. .88 88 .8P !// //! 88888888P dP' 'dP dP '88888' Y88888P' 8888888P !// //!===============================================================!// template constexpr std::pair extgcd(const T a, const T b) { if (b == 0) { return std::pair{1, 0}; } const auto p = extgcd(b, a % b); return {p.second, p.first - p.second * (a / b)}; } template constexpr T inverse(const T a, const T mod) { return (mod + extgcd((mod + a % mod) % mod, mod).first % mod) % mod; } //!========================================================!// //! 8888ba.88ba dP dP dP !// //! 88 '8b '8b 88 88 88 !// //! 88 88 88 .d8888b. .d888b88 88 88d888b. d8888P !// //! 88 88 88 88' '88 88' '88 88 88' '88 88 !// //! 88 88 88 88. .88 88. .88 88 88 88 88 !// //! dP dP dP '88888P' '88888P8 dP dP dP dP !// //!========================================================!// template class ModInt { private: uint v; static uint norm(const uint& x) { return x < mod ? x : x - mod; } static ModInt make(const uint& x) { ModInt m; return m.v = x, m; } static ModInt power(ModInt x, ll n) { ModInt ans = 1; for (; n; n >>= 1, x *= x) { if (n & 1) { ans *= x; } } return ans; } static ModInt inv(const ModInt& x) { return ModInt{inverse((ll)x.v, (ll)mod)}; } public: ModInt() : v{0} {} ModInt(const ll val) : v{norm(uint(val % (ll)mod + (ll)mod))} {} ModInt(const ModInt& n) : v{n()} {} explicit operator bool() const { return v != 0; } ModInt& operator=(const ModInt& n) { return v = n(), (*this); } ModInt& operator=(const ll val) { return v = norm(uint(val % (ll)mod + (ll)mod)), (*this); } ModInt operator+() const { return *this; } ModInt operator-() const { return make(norm(mod - v)); } ModInt operator+(const ModInt& val) const { return make(norm(v + val())); } ModInt operator-(const ModInt& val) const { return make(norm(v + mod - val())); } ModInt operator*(const ModInt& val) const { return make((uint)((ll)v * val() % (ll)mod)); } ModInt operator/(const ModInt& val) const { return *this * inv(val()); } ModInt& operator+=(const ModInt& val) { return *this = *this + val; } ModInt& operator-=(const ModInt& val) { return *this = *this - val; } ModInt& operator*=(const ModInt& val) { return *this = *this * val; } ModInt& operator/=(const ModInt& val) { return *this = *this / val; } ModInt operator+(const ll val) const { return ModInt{v + val}; } ModInt operator-(const ll val) const { return ModInt{v - val}; } ModInt operator*(const ll val) const { return ModInt{(ll)v * (val % mod)}; } ModInt operator/(const ll val) const { return ModInt{(ll)v * inv(val)}; } template ModInt operator^(const I n) const { return power(v, n); } ModInt& operator+=(const ll val) { return *this = *this + val; } ModInt& operator-=(const ll val) { return *this = *this - val; } ModInt& operator*=(const ll val) { return *this = *this * val; } ModInt& operator/=(const ll val) { return *this = *this / val; } template ModInt& operator^=(const I n) { return (*this) = ((*this) ^ n); } bool operator==(const ModInt& val) const { return v == val.v; } bool operator!=(const ModInt& val) const { return not(*this == val); } bool operator==(const ll val) const { return v == norm(uint((ll)mod + val % (ll)mod)); } bool operator!=(const ll val) const { return not(*this == val); } uint operator()() const { return v; } }; template inline ModInt operator+(const ll val, const ModInt& n) { return n + val; } template inline ModInt operator-(const ll val, const ModInt& n) { return ModInt{val - (ll)n()}; } template inline ModInt operator*(const ll val, const ModInt& n) { return n * val; } template inline ModInt operator/(const ll val, const ModInt& n) { return ModInt(val) / n; } template inline bool operator==(const ll val, const ModInt& n) { return n == val; } template inline bool operator!=(const ll val, const ModInt& n) { return not(val == n); } template inline std::istream& operator>>(std::istream& is, ModInt& n) { uint v; return is >> v, n = v, is; } template std::ostream& operator<<(std::ostream& os, const ModInt& n) { return (os << n()); } //!============================================================================!// //! 8888ba.88ba dP a88888b. dP !// //! 88 '8b '8b 88 d8' '88 88 !// //! 88 88 88 .d8888b. .d888b88 88 .d8888b. 88d8b.d8b. 88d888b. !// //! 88 88 88 88' '88 88' '88 88 88' '88 88''88''88 88' '88 !// //! 88 88 88 88. .88 88. .88 Y8. .88 88. .88 88 88 88 88. .88 !// //! dP dP dP '88888P' '88888P8 Y88888P' '88888P' dP dP dP 88Y8888' !// //!============================================================================!// template class ModComb { public: ModComb(const std::size_t N) : f(N + 1, ModInt(1)), in(N + 1, ModInt(1)), invf(N + 1, ModInt(1)) { for (uint i = 2; i <= N; i++) { f[i] = f[i - 1] * i, in[i] = (mod - (mod / i)) * in[mod % i], invf[i] = invf[i - 1] * in[i]; } } ModInt fact(const std::size_t N) const { return f[N]; } ModInt inv(const std::size_t N) const { return in[N]; } ModInt invFact(const std::size_t N) const { return invf[N]; } ModInt perm(const std::size_t N, const std::size_t K) const { return N > f.size() or K > N ? ModInt(0) : f[N] * invf[N - K]; } ModInt comb(const std::size_t N, const std::size_t K) const { return N > f.size() or K > N ? ModInt(0) : f[N] * invf[K] * invf[N - K]; } private: std::vector> f, in, invf; }; template struct Complex { F x, y; Complex() : x(0), y(0) {} Complex(const F& s) : x(std::cos(s)), y(std::sin(s)) {} Complex(const F x, const F y) : x(x), y(y) {} Complex operator-() const { return Complex(-x, -y); } Complex operator+(const Complex& c) const { return Complex{x + c.x, y + c.y}; } Complex operator-(const Complex& c) const { return Complex{x - c.x, y - c.y}; } Complex operator*(const Complex& c) const { return Complex{x * c.x - y * c.y, x * c.y + y * c.x}; } Complex operator*(const F& r) const { return Complex{x * r, y * r}; } Complex& operator+=(const Complex& c) { return this->x += c.x, this->y += c.y, *this; } Complex& operator-=(const Complex& c) { return this->x -= c.x, this->y -= c.y, *this; } Complex& operator*=(const Complex& c) { return *this = *this * c; } Complex& operator*=(const F& r) { return this->x *= r, this->y *= r, *this; } Complex conj() const { return Complex{x, -y}; } }; //!==================================!// //! 88888888b 88888888b d888888P !// //! 88 88 88 !// //! a88aaaa a88aaaa 88 !// //! 88 88 88 !// //! 88 88 88 !// //! dP dP dP !// //!==================================!// template class FFT { private: static constexpr std::size_t L = 30; public: FFT() = delete; static void fft(std::vector>& a, const std::size_t lg, const bool rev) { static std::vector> root[L]; const std::size_t N = a.size(); assert((1UL << lg) == N); if (root[lg].empty()) { root[lg].reserve(N), root[lg].resize(N); for (std::size_t i = 0; i < N; i++) { root[lg][i] = Complex(PI * F(2 * i) / F(N)); } } std::vector> tmp(N); for (std::size_t w = (N >> 1); w > 0; w >>= 1) { for (std::size_t y = 0; y < (N >> 1); y += w) { const Complex r = rev ? root[lg][y].conj() : root[lg][y]; for (std::size_t x = 0; x < w; x++) { const auto u = a[y << 1 | x], v = a[y << 1 | x | w] * r; tmp[y | x] = u + v, tmp[y | x | (N >> 1)] = u - v; } } std::swap(tmp, a); } } template static std::vector simpleConvolute(const std::vector& a, const std::vector& b) { const std::size_t need = a.size() + b.size() - 1, lg = clog(need), N = 1UL << lg; std::vector> A(N), B(N); for (std::size_t i = 0; i < a.size(); i++) { A[i] = Complex{F(a[i]), 0}; } for (std::size_t i = 0; i < b.size(); i++) { B[i] = Complex{F(b[i]), 0}; } fft(A, lg, false), fft(B, lg, false); for (std::size_t i = 0; i < N; i++) { A[i] *= B[i] * ((F)1 / (F)N); } fft(A, lg, true); std::vector ans(need); for (std::size_t i = 0; i < need; i++) { ans[i] = T(std::round(A[i].x)); } return ans; } template static std::vector convolute(const std::vector& a, const std::vector& b) { constexpr std::size_t V = 30; constexpr std::size_t S = (V + K - 1) / K; const std::size_t need = a.size() + b.size() - 1, lg = clog(need), N = 1UL << lg; std::vector> A[K], B[K], tmp(N); for (std::size_t i = 0; i < K; i++) { A[i].reserve(N), A[i].resize(N), B[i].reserve(N), B[i].resize(N); std::fill(tmp.begin() + std::min(a.size(), b.size()), tmp.end(), Complex{}); for (std::size_t j = 0; j < a.size(); j++) { tmp[j].x = F((a[j] >> (S * i)) & ((1 << S) - 1)); } for (std::size_t j = 0; j < b.size(); j++) { tmp[j].y = F((b[j] >> (S * i)) & ((1 << S) - 1)); } fft(tmp, lg, false); for (std::size_t j = 0; j < N; j++) { tmp[j] *= F(0.5); } for (std::size_t j = 0; j < N; j++) { const std::size_t k = j == 0 ? 0UL : N - j; A[i][j] = Complex{tmp[j].x + tmp[k].x, tmp[j].y - tmp[k].y}, B[i][j] = Complex{tmp[j].y + tmp[k].y, -tmp[j].x + tmp[k].x}; } } std::vector> Z[K]; for (std::size_t i = 0; i < K; i++) { Z[i].reserve(N), Z[i].resize(N); } for (std::size_t a = 0; a < K; a++) { for (std::size_t b = 0; b < K; b++) { for (std::size_t i = 0; i < N; i++) { if (a + b < K) { Z[a + b][i] += A[a][i] * B[b][i]; } else { Z[a + b - K][i] += A[a][i] * B[b][i] * Complex(0, 1); } } } } for (std::size_t i = 0; i < K; i++) { fft(Z[i], lg, true); } std::vector ans(need); T base = 1; for (std::size_t k = 0; k < 2 * K - 1; k++, base *= (1LL << S)) { for (std::size_t i = 0; i < need; i++) { if (k < K) { ans[i] += base * T(std::round(Z[k][i].x / F(N))); } else { ans[i] += base * T(std::round(Z[k - K][i].y / F(N))); } } } return ans; } template static std::vector> convolute(const std::vector>& a, const std::vector>& b) { constexpr std::size_t V = 30; constexpr std::size_t S = (V + K - 1) / K; const std::size_t need = a.size() + b.size() - 1, lg = clog(need), N = 1UL << lg; std::vector> A[K], B[K], tmp(N); for (std::size_t i = 0; i < K; i++) { A[i].reserve(N), A[i].resize(N), B[i].reserve(N), B[i].resize(N); std::fill(tmp.begin() + std::min(a.size(), b.size()), tmp.end(), Complex{}); for (std::size_t j = 0; j < a.size(); j++) { tmp[j].x = F((a[j]() >> (S * i)) & ((1 << S) - 1)); } for (std::size_t j = 0; j < b.size(); j++) { tmp[j].y = F((b[j]() >> (S * i)) & ((1 << S) - 1)); } fft(tmp, lg, false); for (std::size_t j = 0; j < N; j++) { tmp[j] *= F(0.5); } for (std::size_t j = 0; j < N; j++) { const std::size_t k = j == 0 ? 0UL : N - j; A[i][j] = Complex{tmp[j].x + tmp[k].x, tmp[j].y - tmp[k].y}, B[i][j] = Complex{tmp[j].y + tmp[k].y, -tmp[j].x + tmp[k].x}; } } std::vector> Z[K]; for (std::size_t i = 0; i < K; i++) { Z[i].reserve(N), Z[i].resize(N); } for (std::size_t a = 0; a < K; a++) { for (std::size_t b = 0; b < K; b++) { for (std::size_t i = 0; i < N; i++) { if (a + b < K) { Z[a + b][i] += A[a][i] * B[b][i]; } else { Z[a + b - K][i] += A[a][i] * B[b][i] * Complex(0, 1); } } } } for (std::size_t i = 0; i < K; i++) { fft(Z[i], lg, true); } std::vector> ans(need); ModInt base = 1; for (std::size_t k = 0; k < 2 * K - 1; k++, base *= (1LL << S)) { for (std::size_t i = 0; i < need; i++) { if (k < K) { ans[i] += int((base * ll(std::round(Z[k][i].x / F(N))))()); } else { ans[i] += int((base * ll(std::round(Z[k - K][i].y / F(N))))()); } } } return ans; } }; //!===================================!// //! 888888ba dP !// //! 88 '8b 88 !// //! a88aaaa8P' .d8888b. 88 dP dP !// //! 88 88' '88 88 88 88 !// //! 88 88. .88 88 88. .88 !// //! dP '88888P' dP '8888P88 !// //! .88 !// //! d8888P !// //!===================================!// template class Poly { public: using mint = ModInt; Poly rev(const std::size_t l) const { std::vector ans = v; ans.resize(l); std::reverse(ans.begin(), ans.end()); return Poly(ans); } Poly() : v(0) {} Poly(const mint& r) : v{r} { shrink(); } Poly(const std::vector& v) : v{v} { shrink(); } Poly(const std::initializer_list&& list) : v{list} { shrink(); } std::vector operator()() const { return v; } mint& operator[](const std::size_t i) { return v[i]; } const mint& operator[](const std::size_t i) const { return v[i]; } mint at(const std::size_t i) const { return i < size() ? v[i] : mint(0); } Poly operator-() const { std::vector ans = v; for (auto& e : ans) { e = -e; } return Poly(ans); } Poly operator+(const Poly& p) const { const std::size_t sz = std::max(p.size(), size()); std::vector ans(sz); for (std::size_t i = 0; i < sz; i++) { ans[i] = at(i) + p.at(i); } return Poly(ans); } Poly operator-(const Poly& p) const { const std::size_t sz = std::max(p.size(), size()); std::vector ans(sz); for (std::size_t i = 0; i < sz; i++) { ans[i] = at(i) - p.at(i); } return Poly(ans); } Poly operator*(const Poly& p) const { return p.size() == 0 or size() == 0 ? Poly() : Poly(multiply(v, p())); } Poly operator*(const mint& r) const { std::vector ans = v; for (auto& e : ans) { e *= r; } return Poly(ans); } Poly operator/(const mint& r) const { std::vector ans = v; for (auto& e : ans) { e /= r; } return Poly(ans); } Poly operator<<(const std::size_t s) const { const std::size_t N = size(); if (N <= s) { return Poly(); } std::vector ans(N - s); for (std::size_t i = 0; i < N - s; i++) { ans[i] = v[i + s]; } return Poly(ans); } Poly operator>>(const std::size_t s) const { const std::size_t N = size(); if (N == 0) { return Poly(); } std::vector ans(N + s); for (std::size_t i = 0; i < N; i++) { ans[i + s] = v[i]; } return Poly(ans); } Poly operator/(const Poly& p) const { return div(p); } Poly operator%(const Poly& p) const { return rem(p); } Poly& operator+=(const Poly& p) { return *this = (*this + p); } Poly& operator-=(const Poly& p) { return *this = (*this - p); } Poly& operator*=(const Poly& p) { return *this = Poly(multiply(v, p())); } Poly& operator*=(const mint& r) { for (auto& e : v) { e *= r; } } Poly& operator/=(const mint& r) { for (auto& e : v) { e /= r; } } Poly& operator>>=(const std::size_t s) { return *this = (*this >> s); } Poly& operator<<=(const std::size_t s) { return *this = (*this << s); } Poly& operator/=(const Poly& p) { return *this = div(p); } Poly& operator%=(const Poly& p) { return *this = rem(p); } Poly rem(const std::size_t k) const { return size() <= k ? *this : Poly(std::vector(v.begin(), v.begin() + k)); } Poly rem(const Poly& q) const { return *this - div(q) * q; } Poly rem(const Poly& q, const Poly& iq, const std::size_t B) { return *this - q * ((*this * iq) >> (B - 1)); } Poly inv(const std::size_t k) const { Poly q(std::vector(1, mint(1) / v[0])); const auto T = Poly(std::vector{2}); for (std::size_t i = 1, j = 0; j < k; j++, i *= 2) { q = (q * (T - rem(2 * i) * q)).rem(2 * i); } return q; } template Poly power(const I k) const { using mint = ModInt; const std::size_t B = size() * 2 - 1; const auto q = pseudoInv(B); Poly ans(std::vector{1}); const std::size_t D = log2p1(k); for (std::size_t i = 0; i < D; i++) { if (k & (1LL << (D - i - 1))) { ans = (ans << 1).rem(*this, q, B); } if (D - i - 1) { ans = (ans * ans).rem(*this, q, B); } } return ans; } std::size_t size() const { return v.size(); } friend std::ostream& operator<<(std::ostream& os, const Poly& p) { if (p.size() == 0) { return os << "0"; } for (std::size_t i = 0; i < p.size(); i++) { os << (i != 0 ? "+" : "") << p[i] << (i != 0 ? i == 1 ? "X" : "X^" + std::to_string(i) : ""); } return os; } private: Poly div(const Poly& q) const { assert(q.size() > 0); if (size() < q.size()) { return Poly(); } const std::size_t N = size(); const auto iq = q.pseudoInv(N); return (*this * iq) >> (N - 1); } void shrink() { for (; not v.empty() and v.back() == 0; v.pop_back()) {} } static std::vector multiply(const std::vector& a, const std::vector& b) { return FFT::convolute(a, b); } Poly pseudoInv(const std::size_t B) const { const std::size_t N = size(); return rev(N).inv(B + 2 > N ? clog(B - N + 2) : 0).rev(B + 1 - N); } std::vector v; }; //!============================================!// //! 8888ba.88ba oo !// //! 88 '8b '8b !// //! 88 88 88 .d8888b. dP 88d888b. !// //! 88 88 88 88' '88 88 88' '88 !// //! 88 88 88 88. .88 88 88 88 !// //! dP dP dP '88888P8 dP dP dP !// //!============================================!// using mint = ModInt; int main() { int N; uint B; std::cin >> N >> B; assert(1 <= N and N <= 200000); assert(1 <= B and B < MOD); std::vector c(N + 1, 0); for (int i = 0; i < N; i++) { int S; std::cin >> S; assert(0 <= S and S < N); c[S]++; } std::vector C = c; for (int i = N - 1; i >= 0; i--) { C[i] += C[i + 1]; } ModComb modcomb(N); std::queue> Q; for (int i = 0; i < N; i++) { if (c[N - i - 1] == 0) { continue; } const mint alpha = modcomb.fact(C[N - i - 1]) * modcomb.invFact(C[N - i]) * c[N - i - 1] * modcomb.inv(C[N - i - 1]); const mint beta = modcomb.fact(C[N - i - 1]) * modcomb.invFact(C[N - i]) * C[N - i] * modcomb.inv(C[N - i - 1]); Q.push({beta, alpha}); } while (Q.size() > 1) { const auto f1 = Q.front(); Q.pop(); const auto f2 = Q.front(); Q.pop(); Q.push(f1 * f2); } const auto P = Q.front(); mint ans = 0; mint base = 1; for (int i = 0; i <= N; i++, base *= B) { ans += i * base * P.at(i); } std::cout << ans << std::endl; return 0; }