显然每个人的最少次数是左边大于他的人数和右边小于他的人数,两次树状数组扫一下就ok了,由于H=1e6,所以先离散化一下,再套树状数组。
Code:
#include <bits/stdc++.h>
#define ll long long
using namespace std;
ll calc(int k)
{
return 1LL * k * (k + 1) / 2;
}
int a[100005], c[100005], cnt[100005], t[100005];
int lowbit(int k)
{
return k & -k;
}
void update(int k, int x)
{
while (k < 100005)
{
c[k] += x;
k += lowbit(k);
}
}
int query(int k)
{
int ans = 0;
while (k)
{
ans += c[k];
k -= lowbit(k);
}
return ans;
}
int main()
{
int n;
scanf("%d", &n);
for (int i = 0; i < n; i++)
{
scanf("%d", &a[i]);
t[i] = a[i];
}
sort(t, t + n);
int num = unique(t, t + n) - t;
for (int i = 0; i < n; i++)
a[i] = lower_bound(t, t + num, a[i]) - t + 1;
for (int i = 0; i < n; i++)
{
cnt[i] += i - query(a[i]);
update(a[i], 1);
}
memset(c, 0, sizeof c);
for (int i = n - 1; i >= 0; i--)
{
update(a[i], 1);
cnt[i] += query(a[i] - 1);
}
ll ans = 0;
for (int i = 0; i < n; i++)
ans += calc(cnt[i]);
printf("%lld", ans);
}