Created
September 9, 2012 21:50
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Euler #47
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| #include <vector> | |
| #include <iostream> | |
| #include <algorithm> | |
| class Primegen | |
| { | |
| std::vector<int> primes; //store all the primes we've generated so far | |
| void nextprime(void); //add one new prime to the end of the list | |
| public: | |
| Primegen(void); //constructor | |
| int operator [] (unsigned int n); //return the nth prime number | |
| bool isprime(int n); //return true if n is prime | |
| }; | |
| void Primegen::nextprime() | |
| { | |
| int current=primes.back(); | |
| bool isprime=true; | |
| do | |
| { | |
| current++; | |
| isprime=true; //start by assuming number is prime | |
| for(unsigned int i=0; i<primes.size() && isprime && primes[i]*primes[i] <= current; i++) | |
| { | |
| if(current%primes[i]==0) | |
| { | |
| isprime=false; //number is not prime if it has a factor | |
| } | |
| } | |
| }while(!isprime); | |
| primes.push_back(current); | |
| } | |
| Primegen::Primegen(void) | |
| { | |
| primes.push_back(2); //seed the generator with a single prime | |
| } | |
| int Primegen::operator [] (unsigned int n) | |
| { | |
| while(n>=primes.size()) | |
| { | |
| nextprime(); | |
| } | |
| return primes[n]; | |
| } | |
| bool Primegen::isprime(int n) | |
| { | |
| while(n>primes.back()) //make sure we have at least one prime bigger than n | |
| { | |
| nextprime(); | |
| } | |
| return std::binary_search(primes.begin(), primes.end(), n); | |
| } | |
| int distinct_factors(Primegen &primes, int n) | |
| { | |
| int ret=0; | |
| int p=primes[0]; | |
| for(int i=0; primes[i]*primes[i] < n; p=primes[++i]) | |
| { | |
| if(n%p==0) | |
| { | |
| ret++; | |
| while(n%p==0) | |
| { | |
| n/=p; | |
| } | |
| } | |
| } | |
| if(n!=1) | |
| { | |
| ret+=1; | |
| } | |
| return ret; | |
| } | |
| int main() | |
| { | |
| Primegen primes; | |
| int streak=0, i; | |
| for(i=3; streak<4; i++) | |
| { | |
| if(distinct_factors(primes, i) >= 4) | |
| { | |
| streak++; | |
| } | |
| else | |
| { | |
| streak=0; | |
| } | |
| } | |
| std::cout << i-4 << std::endl; | |
| return 0; | |
| } |
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I believe that 2 * 2 * 2 * 16741 = 133928 is the correct solution for problem 47.