@zzzc18
2017-12-16T10:18:18.000000Z
字数 689
阅读 1597
BIT
模板库
树状数组求逆序对;
直接按权值排序离散,将位置插入树状数组,该位置的前缀和就是权值比其小而比它大的数的个数
#include<cstdio>
#include<algorithm>
#define MAXN 50000
#define lowbit(x) ((x)&(-x))
int n;
struct data{
int val,id;
}num[MAXN];
bool cmp(const data &x,const data &y){
return x.val<y.val;
}
class Binary_Tree{
private:
int tree[MAXN];
public:
void insert(int pos,int val){
int i;
for(i=pos;i<=n;i+=lowbit(i))
tree[i]+=val;
}
int getnum(int pos){
int i;int ans=0;
for(i=pos;i;i-=lowbit(i))
ans+=tree[i];
return ans;
}
}BIT;
void solve(){
int i;int sum=0;
for(i=1;i<=n;i++){
BIT.insert(num[i].id,1);
sum+=i-BIT.getnum(num[i].id);
}
printf("%d\n",sum);
}
int main(){
scanf("%d",&n);
int i;
for(i=1;i<=n;i++){
scanf("%d",&num[i].val);
num[i].id=i;
}
std::sort(num+1,num+n+1,cmp);
solve();
return 0;
}