区间第K大
对于每个数我们考虑它对答案的贡献是多少，它在哪些区间里是第k大的
这道题我们用双向链表实现
for(i = 1~n)，把值为i的位置从链表里面删除
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| #include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 501000;
int l[N],r[N],pos[N],le[N],ri[N];
int main()
{
int n,k;
cin>>n>>k;
for(int i = 1;i<=n;i++)
{
int p;
cin>>p;
pos[p] = i;
}
for(int i = 0;i<=n+1;i++)
{
l[i] = max(i-1,0);
r[i] = min(i+1,n+1);
}
ll ans = 0;
for(int i = 1;i<=n;i++)
{
int x = pos[i];
le[0] = x;
for(int j = 1;j<=k;j++)
{
x = l[x];
le[j] = x;
}
x = pos[i];
ri[0] = x;
for(int j = 1;j<=k;j++)
{
x = r[x];
ri[j] = x;
}
x = pos[i];
l[r[x]] = l[x];
r[l[x]] = r[x];
ll seg = 0;
for(int j = 1;j<=k;j++)
{
seg += 1ll*(le[j-1]-le[j])*(ri[k-j+1]-ri[k-j]);
}
ans += seg*i;
}
cout<<ans<<endl;
return 0;
}
|
