Project Euler：Problem 41 Pandigital prime

Posted 2020-09-30 blfbuaa

We shall say that an n-digit number is pandigital if it makes use of all the digits 1 to n exactly once. For example, 2143 is a 4-digit pandigital and is also prime.

What is the largest n-digit pandigital prime that exists?

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

int res = 0;

bool prim(int a)
{
	for (int i = 2; i*i <= a; i++)
	{
		if (a%i == 0)
			return false;
	}
	return true;
}

void perm(int list[], int n, int k)
{
	int temp1, temp2;
	if (n == 1)
	{
		int sum = 0;
		for (int i = k; i > 0; i--)
			sum = sum * 10 + list[i];
		if (prim(sum)&&sum > res)
			res = sum;
	}
	else
	for (int i = 1; i <= n; i++)
	{

		temp1 = list[i];
		list[i] = list[n];
		list[n] = temp1;

		perm(list, n - 1, k);

		temp2 = list[i];
		list[i] = list[n];
		list[n] = temp2;
	}

}

int main()
{
	for (int j = 9; j >= 1; j--)
	{
		int list[200];
		for (int i = 1; i <= j; i++)
			list[i] = i;
		perm(list, j, j);
	}
	cout << res << endl;
	system("pause");

	return 0;
}