Codeforces Beta Round #50 C. First Digit Law(概率dp,好题)-白红宇的个人博客

Codeforces Beta Round #50 C. First Digit Law(概率dp,好题)

发布日期：2021-11-12 00:25:49 浏览次数：9 分类：技术文章

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In the probability theory the following paradox called Benford's law is known: "In many lists of random numbers taken from real sources, numbers starting with digit 1 occur much more often than numbers starting with any other digit" (that's the simplest form of the law).

Having read about it on Codeforces, the Hedgehog got intrigued by the statement and wishes to thoroughly explore it. He finds the following similar problem interesting in particular: there are N random variables, the i-th of which can take any integer value from some segment [Li;Ri] (all numbers from this segment are equiprobable). It means that the value of the i-th quantity can be equal to any integer number from a given interval [Li;Ri] with probability 1 / (Ri - Li + 1).

The Hedgehog wants to know the probability of the event that the first digits of at least K% of those values will be equal to one. In other words, let us consider some set of fixed values of these random variables and leave only the first digit (the MSD — most significant digit) of each value. Then let's count how many times the digit 1 is encountered and if it is encountered in at least K per cent of those N values, than such set of values will be called a good one. You have to find the probability that a set of values of the given random variables will be a good one.

Input

The first line contains number N which is the number of random variables (1 ≤ N ≤ 1000). Then follow N lines containing pairs of numbers Li, Ri, each of whom is a description of a random variable. It is guaranteed that 1 ≤ Li ≤ Ri ≤ 1018.

The last line contains an integer K (0 ≤ K ≤ 100).

All the numbers in the input file are integers.

Please, do not use %lld specificator to read or write 64-bit integers in C++. It is preffered to use cin (also you may use %I64d).

Output

Print the required probability. Print the fractional number with such a precision that the relative or absolute error of the result won't exceed 10 - 9.

     Examples 
       input 
11 250
       output 
0.500000000000000
       input 
21 29 1150
       output 
0.833333333333333

题意：

给出n个数，然后给出n行，每行li，ri，代表第i个数在区间[li,ri]中，求一个概率使得这n个数中有K%的数是1开头的。

题解：

首先，可以先求出每个区间以1开头的数有多少个，即每个区间满足条件的概率pi，要使得有K%的区间满足条件，这个该怎么求呢？

我一开始以为是概率方面的题，想了一堆容斥什么的，发现都不可做，最后看了官方题解。

题解是用概率dp做的（妙啊！），d[i][j]表示前i个数有j个满足条件的概率。

#include
   
    #include
    
     #include
     
      #include
      
       #include
       
        #include
        
         #include
         
          using namespace std;#define rep(i,a,n) for (int i=a;i
          
           =a;i--)#define pb push_back#define fi first#define se secondtypedef vector
           
             VI;typedef long long ll;typedef pair
            
              PII;const int inf=0x3fffffff;const ll mod=1000000007;const int maxn=1000+10;ll f[30];ll pow(int x){ ll t=10,ans=1; while(x) { if(x&1) ans*=t; t=t*t; x/=2; } return ans;}ll cal(ll x){ int cnt=0; ll t=x; ll las=0; while(t) { las=t; cnt++; t/=10; } if(las!=1) return f[cnt]; else return x-pow(cnt-1)+1+f[cnt-1];}double p[maxn];double d[maxn][maxn];int main(){ f[1]=1; ll q=1; rep(i,2,20) { q*=10; f[i]=f[i-1]+q; } int n; scanf("%d",&n); rep(i,1,n+1) { ll l,r; scanf("%lld%lld",&l,&r); ll ans=cal(r)-cal(l-1); p[i]=1.0*ans/(r-l+1); } int k; scanf("%d",&k); d[0][0]=1; rep(i,1,n+1) rep(j,0,i+1) { if(j>0) d[i][j]=d[i-1][j-1]*p[i]+d[i-1][j]*(1-p[i]); else d[i][j]=d[i-1][j]*(1-p[i]); } double kk=1.0*k/100; double ans=0; rep(i,0,n+1) { double t=1.0*i/n; if(t>=kk) ans+=d[n][i]; } printf("%.9lf\n",ans); return 0;}

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