3509: [CodeChef] COUNTARI

题意：统计满足\(i<j<k, 2*a[j] = a[i] + a[k]\)的个数

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\(2*a[j]\)不太好处理，暴力fft不如直接暴力

考虑分块

每个块用生成函数统计j在块中ik在两边的块中的

有两个在块中或者三个都在暴力统计，实时维护两边权值出现次数

#include <iostream>

#include <cstdio>

#include <cstring>

#include <algorithm>

#include <cmath>

#include <ctime>

using namespace std;

typedef long long ll;

const int N = (1<<16) + 5, M = 1e5+5;

const double PI = acos(-1.0);

inline int read() {

    char c=getchar(); int x=0,f=1;

    while(c<'0'||c>'9') {if(c=='-')f=-1;c=getchar();}

    while(c>='0'&&c<='9') {x=x*10+c-'0';c=getchar();}

    return x*f;

}

struct meow{

    double x, y;

    meow(double a=0, double b=0):x(a), y(b){}

};

meow operator +(meow a, meow b) {return meow(a.x+b.x, a.y+b.y);}

meow operator -(meow a, meow b) {return meow(a.x-b.x, a.y-b.y);}

meow operator *(meow a, meow b) {return meow(a.x*b.x-a.y*b.y, a.x*b.y+a.y*b.x);}

meow conj(meow a) {return meow(a.x, -a.y);}

typedef meow cd;

namespace fft {

	int n, rev[N];

	cd omega[N], omegaInv[N];

	void init(int lim) {

		n = 1; while(n < lim) n <<= 1;

		for(int i=0; i<n; i++) {

			omega[i] = cd(cos(2*PI/n*i), sin(2*PI/n*i));

			omegaInv[i] = conj(omega[i]);

		}

	}

	void dft(cd *a, int n, int flag) {

		cd *w = flag == 1 ? omega : omegaInv;

		int k = 0; while((1<<k) < n) k++;

		for(int i=0; i<n; i++) {

			rev[i] = (rev[i>>1]>>1) | ((i&1)<<(k-1));

			if(i < rev[i]) swap(a[i], a[rev[i]]);

		}

		for(int l=2; l<=n; l<<=1) {

			int m = l>>1;

			for(cd *p = a; p != a+n; p += l)

				for(int k=0; k<m; k++) {

					cd t = w[n/l*k] * p[k+m];

					p[k+m] = p[k] - t;

					p[k] = p[k] + t;

				}

		}

		if(flag == -1) for(int i=0; i<n; i++) a[i].x /= n;

	}

	void mul(cd *a, cd *b) {

		dft(a, n, 1); dft(b, n, 1);

		for(int i=0; i<n; i++) a[i] = a[i] * b[i];

		dft(a, n, -1);

	}

}

int n, a[M], c1[N], c2[N], block, m;

cd p[N], q[N];

ll ans;

int main() {

	freopen("in", "r", stdin);

	n=read();

	block = min(n, 8 * (int) sqrt(n));

	for(int i=1; i<=n; i++) a[i] = read(), c2[a[i]]++, m = max(m, a[i]);

	fft::init(m+m+1);

	for(int l=1; l<=n; l+=block) {

		int r = min(n, l+block-1);

		for(int i=l; i<=r; i++) c2[a[i]]--;

		for(int i=l; i<=r; i++) {

			for(int j=i+1; j<=r; j++) {

				int t = (a[j]<<1) - a[i];

				if(t >= 0) ans += c2[t];

				t = (a[i]<<1) - a[j];

				if(t >= 0) ans += c1[t];

			}

			c1[a[i]]++;

		}

		//printf("hi [%d, %d] %lld\n", l, r, ans);

	}

	for(int l=1; l<=n; l+=block) { //printf("l %d\n", l);

		int r = min(n, l+block-1);

		memset(p, 0, sizeof(p)); memset(q, 0, sizeof(q));

		for(int i=1; i<l; i++) p[a[i]].x ++;

		for(int i=r+1; i<=n; i++) q[a[i]].x ++;

		fft::mul(p, q);

		for(int i=l; i<=r; i++) ans += (ll) floor(p[a[i]<<1].x + 0.5);

	}

	printf("%lld", ans);

	//printf("\n%lf", (double) clock() / CLOCKS_PER_SEC);

}