树状数组求逆序对
P1908 逆序对
离散化+树状数组：AC_Code
1 #include <bits/stdc++.h> 2 using namespace std; 3 typedef long long ll; 4 const int maxn=500010; 5 6 int a[maxn],tree[maxn<<2],b[maxn]; 7 int cnt; 8 9 int lowbit(int i){ 10 return i&(-i); 11 } 12 13 void updata(int i,int k){ 14 while( i<=cnt ){ 15 tree[i]+=k; 16 i+=lowbit(i); 17 } 18 } 19 20 ll getsum(int i){ 21 ll res=0; 22 while( i>0 ){ 23 res+=tree[i]; 24 i-=lowbit(i); 25 } 26 return res; 27 } 28 29 int main() 30 { 31 int n; 32 scanf("%d",&n); 33 for(int i=1;i<=n;i++){ 34 scanf("%d",&a[i]); 35 b[i-1]=a[i]; 36 } 37 sort(b,b+n); 38 int reu=unique(b,b+n)-b; 39 cnt=reu; 40 memset(tree,0,sizeof(tree)); 41 ll ans=0; 42 for(int i=1;i<=n;i++){ 43 int f=lower_bound(b,b+reu,a[i])-b+1; 44 ll t=getsum(cnt)-getsum(f); //t=getsum(cnt)-getsum(a[i]); 45 updata(f,1); 46 ans += t; 47 } 48 cout<<ans<<endl; 49 return 0; 50 }
