[hdu5225]逆序对统计

  1 #pragma comment(linker, "/STACK:10240000,10240000")
  2 
  3 #include <iostream>
  4 #include <cstdio>
  5 #include <algorithm>
  6 #include <cstdlib>
  7 #include <cstring>
  8 #include <map>
  9 #include <queue>
 10 #include <deque>
 11 #include <cmath>
 12 #include <vector>
 13 #include <ctime>
 14 #include <cctype>
 15 #include <set>
 16 #include <bitset>
 17 #include <functional>
 18 #include <numeric>
 19 #include <stdexcept>
 20 #include <utility>
 21 
 22 using namespace std;
 23 
 24 #define mem0(a) memset(a, 0, sizeof(a))
 25 #define mem_1(a) memset(a, -1, sizeof(a))
 26 #define lson l, m, rt << 1
 27 #define rson m + 1, r, rt << 1 | 1
 28 #define define_m int m = (l + r) >> 1
 29 #define rep_up0(a, b) for (int a = 0; a < (b); a++)
 30 #define rep_up1(a, b) for (int a = 1; a <= (b); a++)
 31 #define rep_down0(a, b) for (int a = b - 1; a >= 0; a--)
 32 #define rep_down1(a, b) for (int a = b; a > 0; a--)
 33 #define all(a) (a).begin(), (a).end()
 34 #define lowbit(x) ((x) & (-(x)))
 35 #define constructInt4(name, a, b, c, d) name(int a = 0, int b = 0, int c = 0, int d = 0): a(a), b(b), c(c), d(d) {}
 36 #define constructInt3(name, a, b, c) name(int a = 0, int b = 0, int c = 0): a(a), b(b), c(c) {}
 37 #define constructInt2(name, a, b) name(int a = 0, int b = 0): a(a), b(b) {}
 38 #define pchr(a) putchar(a)
 39 #define pstr(a) printf("%s", a)
 40 #define sstr(a) scanf("%s", a)
 41 #define sint(a) scanf("%d", &a)
 42 #define sint2(a, b) scanf("%d%d", &a, &b)
 43 #define sint3(a, b, c) scanf("%d%d%d", &a, &b, &c)
 44 #define pint(a) printf("%d\n", a)
 45 #define test_print1(a) cout << "var1 = " << a << endl
 46 #define test_print2(a, b) cout << "var1 = " << a << ", var2 = " << b << endl
 47 #define test_print3(a, b, c) cout << "var1 = " << a << ", var2 = " << b << ", var3 = " << c << endl
 48 #define mp(a, b) make_pair(a, b)
 49 #define pb(a) push_back(a)
 50 
 51 typedef unsigned int uint;
 52 typedef long long LL;
 53 typedef pair<int, int> pii;
 54 typedef vector<int> vi;
 55 
 56 const int dx[8] = {0, 0, -1, 1, 1, 1, -1, -1};
 57 const int dy[8] = {-1, 1, 0, 0, 1, -1, 1, -1 };
 58 const int maxn = 1e8 + 17;
 59 const int md = 1e9 + 7;
 60 const int inf = 1e9 + 7;
 61 const LL inf_L = 1e18 + 7;
 62 const double pi = acos(-1.0);
 63 const double eps = 1e-6;
 64 
 65 template<class T>T gcd(T a, T b){return b==0?a:gcd(b,a%b);}
 66 template<class T>bool max_update(T &a,const T &b){if(b>a){a = b; return true;}return false;}
 67 template<class T>bool min_update(T &a,const T &b){if(b<a){a = b; return true;}return false;}
 68 template<class T>T condition(bool f, T a, T b){return f?a:b;}
 69 template<class T>void copy_arr(T a[], T b[], int n){rep_up0(i,n)a[i]=b[i];}
 70 int make_id(int x, int y, int n) { return x * n + y; }
 71 
 72 template<int mod>
 73 struct ModInt {
 74     const static int MD = mod;
 75     int x;
 76     ModInt(int x = 0): x(x) { if (x < 0) x += mod; }
 77     int get() { return x; }
 78 
 79     ModInt operator + (const ModInt &that) const { int x0 = x + that.x; return ModInt(x0 < MD? x0 : x0 - MD); }
 80     ModInt operator - (const ModInt &that) const { int x0 = x - that.x; return ModInt(x0 < MD? x0 + MD : x0); }
 81     ModInt operator * (const ModInt &that) const { return ModInt((long long)x * that.x % MD); }
 82     ModInt operator / (const ModInt &that) const { return *this * that.inverse(); }
 83 
 84     ModInt operator += (const ModInt &that) { x += that.x; if (x >= MD) x -= MD; }
 85     ModInt operator -= (const ModInt &that) { x -= that.x; if (x < 0) x += MD; }
 86     ModInt operator *= (const ModInt &that) { x = (long long)x * that.x % MD; }
 87     ModInt operator /= (const ModInt &that) { *this = *this / that; }
 88 
 89     ModInt inverse() const {
 90         int a = x, b = MD, u = 1, v = 0;
 91         while(b) {
 92             int t = a / b;
 93             a -= t * b; std::swap(a, b);
 94             u -= t * v; std::swap(u, v);
 95         }
 96         if(u < 0) u += MD;
 97         return u;
 98     }
 99 
100 };
101 typedef ModInt<md> mint;
102 mint dp[107], fact[107];
103 int n, a[107];
104 bool vis[107];
105 
106 void init() {
107     fact[0] = 1;
108     rep_up1(i, 102) {
109         dp[i] = dp[i - 1] * i + fact[i - 1] * i * (i - 1) / 2;
110         fact[i] = fact[i - 1] * i;
111     }
112 }
113 
114 int main() {
115     //freopen("in.txt", "r", stdin);
116     init();
117     while (cin >> n) {
118         rep_up0(i, n) sint(a[i]);
119         mem0(vis);
120         mint ans = 0;
121         rep_up0(i, n) {
122             int c = 0;
123             rep_up0(j, i) {
124                 for (int k = j + 1; k < i; k ++) {
125                     if (a[j] > a[k]) c ++;
126                 }
127             }
128             rep_up1(j, a[i] - 1) {
129                 if (!vis[j]) {
130                     ans += fact[n - i - 1] * c;
131                     rep_up0(k, i) {
132                         if (a[k] > j) ans += fact[n - i - 1];
133                     }
134                     int sum = 0;
135                     vis[j] = true;
136                     rep_up1(k, n) {
137                         if (!vis[k]) sum ++;
138                         else ans += fact[n - i - 1] * sum;
139                     }
140                     ans += dp[n - i - 1];
141                     vis[j] =false;
142                 }
143             }
144             vis[a[i]] = true;
145         }
146         cout << ans.get() << endl;
147     }
148     return 0;
149 }