CF-995-E. Best Price · znzryb
## 思路讲解
二分是假的，离散化，差分是真的。
还是有一定难度的，一是要想到离散化+差分快速判断该数是否符合条件。第二是要缩小答案范围，注意到，只有a和b才能成为答案（a+1不就差评了？那为什么不选b？—————a-1不就还好评那？那为什么不选a？）
## AC代码
[https://codeforces.com/contest/2051/submission/319412463](https://codeforces.com/contest/2051/submission/319412463)
::: 源代码
```cpp
// Problem: E. Best Price
// Contest: Codeforces - Codeforces Round 995 (Div. 3)
// URL: https://codeforces.com/contest/2051/problem/E
// Memory Limit: 256 MB
// Time Limit: 2000 ms
// by znzryb
//
// Powered by CP Editor (https://cpeditor.org)
#include <bits/stdc++.h>
#define all(vec) vec.begin(),vec.end()
#define CLR(i,a) memset(i,a,sizeof(i))
#define fi first
#define se second
#define pb push_back
#define SZ(a) ((int) a.size())
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define ROF(i, a, b) for (int i = (a); i >= (b); --i)
#define getFro(vec) (vec.empty()?0:vec.front())
#define getBac(vec) (vec.empty()?0:vec.front())
#define debug(var) cerr << #var <<":"<<var<<"\n";
#define DEBUG(variable) \
do { \
std::cerr << #variable << ":"; \
for (const auto& elem : variable) { \
std::cerr << elem << " "; \
} \
std::cerr << "\n"; \
} while (0)
#define uniVec(var) \
do { \
sort(var.begin(),var.end());\
var.resize(unique(var.begin(),var.end())-var.begin());\
} while (0)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef __int128 i128;
typedef pair<ll,ll> pll;
typedef array<ll,2> arr;
typedef double DB;
typedef pair<DB,DB> pdd;
typedef pair<ll,bool> plb;
constexpr ll MAXN=static_cast<ll>(1e6)+10,INF=static_cast<ll>(1e18)+3;
constexpr ll mod=static_cast<ll>(1e9)+7;
constexpr double eps=1e-8;
ll N,M,K,T;
ll ans=0;
arr ab[MAXN];
ll diff[MAXN],Sum[MAXN];
/*
不难注意到，我们只要在合法的价格上，求出
*/
inline void solve(){
cin>>N>>K;
vector<ll> li,Vals,B;
FOR(i,0,3*N+5){
Sum[i]=0;
diff[i]=0;
}
FOR(i,1,N){
cin>>ab[i][0];
li.pb(ab[i][0]+1);
li.pb(ab[i][0]);
Vals.pb(ab[i][0]);
}
FOR(i,1,N){
cin>>ab[i][1];
li.pb(ab[i][1]);
Vals.pb(ab[i][1]);
B.pb(ab[i][1]);
}
sort(all(B));
uniVec(li);
uniVec(Vals);
li.pb(li.back()+1);
// li.insert(-INF,0);
FOR(i,1,N){
// 是从a+1～b这段需要加一
ll beg=lower_bound(all(li),ab[i][0]+1)-li.begin(),
ed=lower_bound(all(li),ab[i][1])-li.begin();
diff[beg]+=1;
diff[ed+1]-=1;
}
Sum[0]=diff[0];
FOR(i,1,3*N){
Sum[i]=Sum[i-1]+diff[i];
}
#ifdef LOCAL
cerr<<"Sum:";
FOR(i,0,2*N){
cerr<<Sum[i]<<" ";
}
cerr<<"\n";
DEBUG(li);
DEBUG(B);
#endif
ans=0;
FOR(i,0,SZ(Vals)-1){
ll val=Vals[i];
ll idx=lower_bound(all(li),val)-li.begin();
#ifdef LOCAL
if(val==1){
debug(Sum[idx]);
debug(K);
}
#endif
if(Sum[idx]<=K){
// B中>=val的值有多少个
ll ind=lower_bound(all(B),val)-B.begin();
ll num=N-ind;
ans=max(num*val,ans);
#ifdef LOCAL
debug(ind);
#endif
}
}
cout<<ans<<"\n";
}
int main()
{
ios::sync_with_stdio(false);
cin.tie(0);cout.tie(0);
cin>>T;
while(T--){
solve();
}
// solve();
return 0;
}
/*
AC
https://codeforces.com/contest/2051/submission/319412463
*/
```
:::
## 心路历程（WA，TLE，MLE……）
三分做法，WA了
[https://codeforces.com/contest/2051/submission/319133918](https://codeforces.com/contest/2051/submission/319133918)
::: 源代码
```cpp
// Problem: E. Best Price
// Contest: Codeforces - Codeforces Round 995 (Div. 3)
// URL: https://codeforces.com/contest/2051/problem/E
// Memory Limit: 256 MB
// Time Limit: 2000 ms
// by znzryb
//
// Powered by CP Editor (https://cpeditor.org)
#include <bits/stdc++.h>
#define all(vec) vec.begin(),vec.end()
#define CLR(i,a) memset(i,a,sizeof(i))
#define fi first
#define se second
#define pb push_back
#define SZ(a) ((int) a.size())
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define ROF(i, a, b) for (int i = (a); i >= (b); --i)
#define getFro(vec) (vec.empty()?0:vec.front())
#define getBac(vec) (vec.empty()?0:vec.front())
#define debug(var) cerr << #var <<":"<<var<<"\n";
#define DEBUG(variable) \
do { \
std::cerr << #variable << ":"; \
for (const auto& elem : variable) { \
std::cerr << elem << " "; \
} \
std::cerr << "\n"; \
} while (0)
#define uniVec(var) \
do { \
sort(var.begin(),var.end());\
var.resize(unique(var.begin(),var.end())-var.begin());\
} while (0)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef __int128 i128;
typedef pair<ll,ll> pll;
typedef array<ll,3> arr;
typedef double DB;
typedef pair<DB,DB> pdd;
typedef pair<ll,bool> plb;
constexpr ll MAXN=static_cast<ll>(1e6)+10,INF=static_cast<ll>(1e18)+3;
constexpr ll mod=static_cast<ll>(1e9)+7;
constexpr double eps=1e-8;
ll N,M,K,T,A[MAXN],B[MAXN];
ll ans=0;
/*
*/
inline ll check(ll x){
ll res=0,neg=0;
#ifdef LOCAL
debug(x)
#endif
FOR(i,1,N){
if(x<=A[i]){
res+=x;
}else if(x>A[i] && x<=B[i]){
neg+=1;
res+=x;
}else{
continue;
}
}
#ifdef LOCAL
debug(neg)debug(res)
#endif
if(neg>K) return 1;
else {ans=max(ans,res);return res;}
}
inline void solve(){
cin>>N>>K;
FOR(i,1,N){
cin>>A[i];
}
ll maxB=0;
FOR(i,1,N){
cin>>B[i];
maxB=max(maxB,B[i]);
}
ll l=0,r=maxB+3;ans=0;
while(r-l>2){
ll mid1=l+(r-l)/3,mid2=r-(r-l)/3;
#ifdef LOCAL
debug(l)debug(r)debug(mid1)debug(mid2)
#endif
if(check(mid1)>check(mid2)){
r=mid2;
}else{
l=mid1;
}
#ifdef LOCAL
cerr << '\n';
#endif
}
cout<<ans<<"\n";
}
int main()
{
ios::sync_with_stdio(false);
cin.tie(0);cout.tie(0);
cin>>T;
while(T--){
solve();
}
// solve();
return 0;
}
/*
*/
```
:::
后面几次WA是因为清空不够多，sum没加够。