백준 1526번 C++

#include <iostream>
#include <queue>

using namespace std;
 
int main(void) {
	int n,num;

	cin >> n;
	queue<int> q;
	if (n >= 4)
		q.push(4);
	if (n >= 7)
		q.push(7);

	while (!q.empty()) {
		num = q.front();
		q.pop();
		
		if(num * 10 + 4 <= n)
			q.push(num * 10 + 4);
		if(num * 10 + 7 <= n)
			q.push(num * 10 + 7);
	}
	cout << num << endl;
	return 0;
}