[CF808B] Average Sleep Time（[强行]树状数组，数学）

题目链接：http://codeforces.com/contest/808/problem/B

题意：n个数，求其中每k个数的和的平均值。

鬼知道我怎么会石乐志上来就用了树状数组。

#include <bits/stdc++.h>
using namespace std;

typedef long long LL;
const int maxn = 200200;
int n, k;
LL a[maxn], bit[maxn];
int lowbit(int x) {return x&(-x);}
void add(int id,LL val) {for(int i=id;i<=k;i+=lowbit(i))bit[i]+=val;}
void sum(int id, LL& ret) {for(int i=id;i>0;i-=lowbit(i))ret+=bit[i];}

int main() {
    // freopen("in", "r", stdin);
    while(~scanf("%d%d",&n,&k)) {
        LL ret = 0;
        memset(bit, 0, sizeof(bit));
        for(int i = 1; i <= n; i++) scanf("%I64d", &a[i]);
        for(int i = 1; i <= k; i++) add(i,a[i]);
        sum(k, ret);
        for(int i = k + 1; i <= n; i++) {
            add(i%k+1,-a[i-k]);
            add(i%k+1,a[i]);
            sum(k, ret);
        }
        printf("%.10f
", (double)ret/(n-k+1));
    }
    return 0;
}

直接用前缀和维护搞。

#include <bits/stdc++.h>
using namespace std;

typedef long long LL;
const int maxn = 200200;
int n, k;
LL a[maxn], s[maxn];

int main() {
    // freopen("in", "r", stdin);
    while(~scanf("%d%d",&n,&k)) {
        memset(s, 0, sizeof(s));
        for(int i = 1; i <= n; i++) {
            scanf("%I64d", &a[i]);
            s[i] = s[i-1] + a[i];
        }
        LL ret = .0;
        for(int i = k; i <= n; i++) ret += s[i] - s[i-k];
        printf("%.10f
", (double)ret / (n - k + 1));
    }
    return 0;
}