思路讲解
BIT+频数数组
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| inline ll query(ll x){
ll idx=li[x];
ll res=0;
while(idx>0){
res+=tr[idx];
idx-=lowbit(idx);
}
return res;
}
cnt+=std::max((i-1-query(A[i])),0LL);
|
AC代码
https://www.luogu.com.cn/record/197653847
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| #include <iostream>
#include <cstring>
#include <algorithm>
#include <deque>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <cmath>
#include <bitset>
#include <iterator>
#include <random>
#include <iomanip>
#include <cctype>
#include <array>
#include <unordered_map>
typedef long long ll;
typedef std::pair<ll,ll> pll;
typedef std::array<ll,3> arr;
const ll MAXN=static_cast<ll>(5e5)+10;
ll N,T,A[MAXN],tr[MAXN];
std::set<ll> ali;
std::unordered_map<ll,ll> li;
inline ll lowbit(ll x){
return x&(-x);
}
inline void add(ll x){
ll idx=li[x];
while (idx<=N) {
tr[idx]+=1;
idx+=lowbit(idx);
}
}
inline ll query(ll x){
ll idx=li[x];
ll res=0;
while(idx>0){
res+=tr[idx];
idx-=lowbit(idx);
}
return res;
}
int main()
{
std::ios::sync_with_stdio(false);
std::cin.tie(0);std::cout.tie(0);
std::cin>>N;
for(int i=1;i<=N;++i){
std::cin>>A[i];
ali.insert(A[i]);
}
ll idx=0;
for(std::set<ll>::iterator it=ali.begin();it!=ali.end();it++){
li[*it]=++idx;
}
ll cnt=0;
for(int i=1;i<=N;++i){
cnt+=std::max((i-1-query(A[i])),0LL);
add(A[i]);
}
std::cout<<cnt<<"\n";
return 0;
}
|
心路历程（WA，TLE，MLE……）
TLE https://www.luogu.com.cn/record/197653726
#include <unordered_map> 这个还是比map要快一点，省了300多ms