CodeForces - 244B（dfs+剪枝，set）

题目链接

题意：求小于n且最多由2个数组成的数的个数

#include
using namespace std;
seta;
int n, x, y;
void dfs(long long num, long long pos) {
	if (num > n || pos > 10)return;
	a.insert(num);
	dfs(num * 10 + x, pos + 1);
	dfs(num * 10 + y, pos + 1);
}
int main() {
	cin >> n;
	for (int i = 0; i < 10; i++)for (int j = i + 1; j < 10; j++) {
		x = i, y = j; dfs(0, 0);
	}
	cout << a.size() - 1 << endl;
	return 0;
}