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题目链接

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 typedef long long ll;
 4 inline ll read(){
 5     int x=0,f=1;char ch=getchar();
 6     while(ch>'9'||ch<'0'){if(ch=='-')f=-1;ch=getchar();}
 7     while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
 8     return x*f;
 9 }
10 
11 /***********************************************************/
12 
13 const int maxn = 1e6+7;
14 double dp[maxn][3];
15 
16 int main(){
17     int n;
18     n = read();
19     for(int i = 1;i <= n;i++){
20         double x;
21         scanf("%lf", &x);
22         dp[i][0] = (dp[i-1][0] + 1)*x;
23         dp[i][1] = (dp[i-1][1] + 2*dp[i-1][0] + 1) * x;
24         dp[i][2] = dp[i-1][2] + (3*dp[i-1][1] + 3*dp[i-1][0] + 1) * x;
25     }
26     printf("%.1lf\n", dp[n][2]);
27     return 0;
28 }

 

posted @ 2018-10-10 16:23  ouyang_wsgwz  阅读(207)  评论(0)    收藏  举报