hdu 5147 Sequence II
http://acm.hdu.edu.cn/showproblem.php?pid=5147
题意：问有多少个这样的四元组(a,b,c,d),满足条件是 1<=a<b<c<d; Aa<Ab; Ac<Ad;
思路：用树状数组求，从右向左求在这个数之前形成多少个逆序数对记录在r数组里面，然后在从左向右求出在输入这个数之前形成多少个逆序数对存在l数组里面，然后枚举b就行；
1 #include <cstdio> 2 #include <cstring> 3 #include <algorithm> 4 #define maxn 50001 5 #define ll long long 6 using namespace std; 7 8 int t; 9 int n; 10 int a[maxn]; 11 int c[maxn]; 12 ll l[maxn],r[maxn],sum[maxn]; 13 14 int lowbit(int x) 15 { 16 return x&-x; 17 } 18 19 void insert(int x,int v) 20 { 21 while(x<maxn) 22 { 23 c[x]+=v; 24 x+=lowbit(x); 25 } 26 } 27 28 ll getsum(int x) 29 { 30 ll ans=0; 31 while(x>0) 32 { 33 ans+=c[x]; 34 x-=lowbit(x); 35 } 36 return ans; 37 } 38 39 int main() 40 { 41 scanf("%d",&t); 42 while(t--) 43 { 44 memset(sum,0,sizeof(sum)); 45 memset(a,0,sizeof(a)); 46 scanf("%d",&n); 47 for(int i=1; i<=n; i++) 48 { 49 scanf("%d",&a[i]); 50 } 51 memset(c,0,sizeof(c)); 52 sum[n+1]=0; 53 for(int i=n; i>=1; i--) 54 { 55 r[i]=getsum(n)-getsum(a[i]); 56 insert(a[i],1); 57 sum[i]=sum[i+1]+r[i]; 58 } 59 memset(c,0,sizeof(c)); 60 for(int i=1; i<=n; i++) 61 { 62 l[i]=getsum(a[i]-1); 63 insert(a[i],1); 64 } 65 ll ans=0; 66 for(int i=2; i<=n; i++) 67 { 68 ans+=(l[i]*sum[i+1]); 69 } 70 printf("%lld\n",ans); 71 } 72 return 0; 73 }
