eight.cpp

#include <iostream>
#include <string>
using namespace std;

long long prod(string str, int index, int digits) {
    long long result = 1;
    for (int i = index; i < index + digits; i++) {
        char d = str[i];
        result *= (long long)(d - '0');
    }
    return result;
}

int main() {
    const char *hugestr = 
    "73167176531330624919225119674426574742355349194934"
    "96983520312774506326239578318016984801869478851843"
    "85861560789112949495459501737958331952853208805511"
    "12540698747158523863050715693290963295227443043557"
    "66896648950445244523161731856403098711121722383113"
    "62229893423380308135336276614282806444486645238749"
    "30358907296290491560440772390713810515859307960866"
    "70172427121883998797908792274921901699720888093776"
    "65727333001053367881220235421809751254540594752243"
    "52584907711670556013604839586446706324415722155397"
    "53697817977846174064955149290862569321978468622482"
    "83972241375657056057490261407972968652414535100474"
    "82166370484403199890008895243450658541227588666881"
    "16427171479924442928230863465674813919123162824586"
    "17866458359124566529476545682848912883142607690042"
    "24219022671055626321111109370544217506941658960408"
    "07198403850962455444362981230987879927244284909188"
    "84580156166097919133875499200524063689912560717606"
    "05886116467109405077541002256983155200055935729725"
    "71636269561882670428252483600823257530420752963450";
    
    string hugenum = hugestr;
    long long max = 0;
    int digits = 13;
    for (int i = 0; i < hugenum.length() - digits; i++) {
        long long p = prod(hugenum, i, digits);
        if (p > max) { max = p; } 
    }
    
    cout << "Largest " << digits << " digit product: " << max << endl;
    return 0;
}