济南学习 Day2 T2 am

 1 #include<cstdio>
 2 #include<cstdlib>
 3 #include<cstring>
 4 #include<algorithm>
 5 using namespace std;
 6 #ifdef unix
 7 #define LL "%lld"
 8 #else
 9 #define LL "%I64d"
10 #endif
11 #define lb(x) ((x)&(-(x)))
12 const int maxn=500010;
13 int n,l,r,y[maxn],z[maxn],w[maxn],x[maxn];
14 bool cmp(int a,int b)
15 {
16     return z[a]>z[b];
17 }
18 void insert(int p)
19 {
20     for (;p<=n;p+=lb(p))
21         w[p]++;
22 }
23 int query(int p)
24 {
25     int ans=0;
26     for (;p;p-=lb(p))
27         ans+=w[p];
28     return ans;
29 }
30 long long solve()
31 {
32     for (int a=1;a<=n;a++)
33         z[a]+=z[a-1];// 前缀和 
34     for (int a=n+1;a>=2;a--)
35         z[a]=z[a-1];// 向后平移一位 
36     z[1]=0;
37     n++;
38     for (int a=1;a<=n;a++)
39         x[a]=a;
40     sort(x+1,x+n+1,cmp);
41     memset(w,0,sizeof(w));
42     long long ans=0;
43     for (int a=1;a<=n;)
44     {
45         int b=a;
46         while (b<=n && z[x[b]]==z[x[a]])
47             b++;
48         for (int c=a;c<b;c++)
49             ans+=query(x[c]-1);
50         for (int c=a;c<b;c++)
51             insert(x[c]);
52         a=b;
53     }
54     n--;
55     return ans;
56 }
57 long long gcd(long long a,long long b)
58 {
59     if (!b) return a;
60     else return gcd(b,a%b);
61 }
62 int main()
63 {
64 
65     scanf("%d%d%d",&n,&l,&r);
66     for (int a=1;a<=n;a++)
67         scanf("%d",&y[a]);
68     for (int a=1;a<=n;a++)
69         z[a]=y[a]-l;
70     long long ans=solve();
71     for (int a=1;a<=n;a++)
72         z[a]=r-y[a];
73     ans+=solve();
74     long long up=ans;
75     long long down=(long long)n*(n+1)/2;
76     up=down-up;
77     long long g=gcd(up,down);
78     up/=g;
79     down/=g;
80     if (up==0) printf("0\n");
81     else
82     {
83         if (down==1) printf("1\n");
84         else printf(LL "/" LL "\n",up,down);
85     }
86 
87     return 0;
88 }