c++ - Project Euler #10 How to pre-calculate sum of primes? -


here question:

the sum of primes below 10 2+3+5+7=17.

find sum of primes not greater given n.

input format : first line contains integer t i.e. number of test cases. next t lines contains integer n.

output format : print value corresponding each test case in seperate line.

constraints : 1≤t≤104 1≤n≤106

https://www.hackerrank.com/contests/projecteuler/challenges/euler010 link question.

so, attempted solve question using sieve of eratosthenes. pre calculated primes below 10^6 given limit n.

6 out of 7 test cases accepted last test case give timeout(tle) .

i read discussion forum , there in order solve question need pre-calculate sums of primes also.

so, tried making array of long long ints , tried storing sums in it. giving me segmentation fault.

so, how supposed precalculate sums of primes?

here code:

#include "header.h"             //max defined 1000000  bool sieve[max + 1];        // false = prime, true = composite  int main(void){     //0 , 1 not primes     sieve[0] = sieve[1] = true;     //input limiting value     int n = max;     //cross out numbers     for(int = 4; <= n; += 2){         sieve[i] = true;     }     //use sieve of eratosthenes     for(int = 3; <= static_cast<int>(sqrt(n)); += 2){              if(sieve[i] == false){                                       for(int j = * i; j <= n; j += i)                               sieve[j] = true;         }     }     long long p, ans = 0;     int t;     std::cin >> t;     while(t--){         std::cin >> p;         for(int = 0; <= p; ++i)             if(sieve[i] == false)                 ans += i;         std::cout << ans << std::endl;         ans = 0;     }     return 0; }

given array of primes prime[n], precomputing sums of primes can done in single for loop this:

int sum[n]; sum[0] = primes[0]; (int = 1 ; < n ; i++) {     sum[i] = prime[i]+sum[i-1]; }

you can use array primes[] running binary search on primes, , picking sum @ same position if number being searched prime, or @ prior position if number not prime.


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