cf 思维 排列组合

B. Tavas and SaDDas
用dfs搜索很快

#include<bits/stdc++.h>
using namespace std;
vector <long long> v;
void f(long long x)
{
	if(x > 1e9) return;
	v.push_back(x);
	f(x*10+4);
	f(x*10+7);
}
int main()
{
	f(0);
	sort(v.begin(), v.end());
	int n;
	cin >> n;
	for(int i = 0 ; i < v.size(); ++i)
		if(v[i] == n){
			cout << i << endl;
		}
	return 0;
}