Project Euler task 44

Pentagonal numbers are generated by the formula, P_n=n(3n1)/2. The first ten pentagonal numbers are:

1, 5, 12, 22, 35, 51, 70, 92, 117, 145, ...

It can be seen that P₄ + P₇ = 22 + 70 = 92 = P₈. However, their difference, 70 22 = 48, is not pentagonal.

Find the pair of pentagonal numbers, P_j and P_k, for which their sum and difference is pentagonal and D = |P_k P_j| is minimised; what is the value of D?

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#include < iostream >

#include < set >

using namespace std;

const int ciMax = 10000000;

int main() {

set< unsigned long > vPenths;

int i = 1;

while (1) {

unsigned long nTemp = i * (3 * i - 1) / 2;

if (nTemp > 10000000)

break;

vPenths.insert(nTemp);

i++;

}

for (set< unsigned long >::iterator it1 = vPenths.begin(); it1 != vPenths.end(); it1++)

for (set< unsigned long >::iterator it2 = it1; it2 != vPenths.end(); it2++) {

if (it1 == it2)

continue;

if (vPenths.find(*it2 - *it1) != vPenths.end() &&

vPenths.find(*it2 + *it1) != vPenths.end()) {

cout << *it1 << "-" << *it2 << "=" << *it2 - *it1 << endl;

break;

}

}

return 0;

}