Codeforces 380E Sereja and Dividing

题面

洛谷传送门

题解

博客

有精度要求所以只用求几十次就差不多了

CODE

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int MAXN = 300005;
struct node {
	int v, id;
	inline bool operator <(const node &o)const {
		return v < o.v || (v == o.v && id < o.id);
	}
}p[MAXN];
int n, pre[MAXN], nxt[MAXN];
int main () {
	scanf("%d", &n);
	for(int i = 1; i <= n; ++i)
		scanf("%d", &p[i].v), p[i].id = i, pre[i] = i-1, nxt[i] = i+1;
	sort(p + 1, p + n + 1);
	double ans = 0;
	for(int i = 1; i <= n; ++i) {
		double l = 0, r = 0, pw = 1;
		int x = p[i].id, lp = x, rp = x;
		for(int j = 1; j <= 50; ++j) {
			pw /= 2;
			if(lp) l += pw*(lp - pre[lp]), lp = pre[lp];
			if(rp<=n) r += pw*(nxt[rp] - rp), rp = nxt[rp];
		}
		ans += 2*l*r*p[i].v;
		nxt[pre[x]] = nxt[x];
		pre[nxt[x]] = pre[x];
	}
	printf("%.15f\n", ans/n/n);
}