[luoguP3608] [USACO17JAN]Balanced Photo平衡的照片（树状数组 + 离散化）

传送门

树状数组裸题

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#define N 100001

using namespace std;

int n, m, ans;
int a[N], b[N], R[N], L[N], c[N];

inline int read()
{
	int x = 0, f = 1;
	char ch = getchar();
	for(; !isdigit(ch); ch = getchar()) if(ch == '-') f = -1;
	for(; isdigit(ch); ch = getchar()) x = (x << 1) + (x << 3) + ch - '0';
	return x * f;
}

inline void add(int x)
{
	for(; x <= m; x += x & -x) c[x]++;
}

inline int query(int x)
{
	int ret = 0;
	for(; x; x -= x & -x) ret += c[x];
	return ret;
}

int main()
{
	int i;
	n = read();
	for(i = 1; i <= n; i++) a[i] = b[i] = read();
	sort(b + 1, b + n + 1);
	m = unique(b + 1, b + n + 1) - b - 1;
	for(i = 1; i <= n; i++)
		a[i] = lower_bound(b + 1, b + m + 1, a[i]) - b;
	for(i = 1; i <= n; i++)
	{
		add(a[i]);
		L[i] = i - query(a[i]);
	}
	memset(c, 0, sizeof(c));
	for(i = n; i >= 1; i--)
	{
		add(a[i]);
		R[i] = (n - i + 1) - query(a[i]);
	}
	for(i = 1; i <= n; i++)
		if(max(L[i], R[i]) > 2 * min(L[i], R[i]))
			ans++;
	printf("%d\n", ans);
	return 0;
}