백준 수 이어쓰기 2 - 이분 탐색

https://velog.io/@embeddedjune/%EC%95%8C%EA%B3%A0%EB%A6%AC%EC%A6%98-%EB%B0%B1%EC%A4%80-%EC%9D%B4%EC%A7%84%ED%83%90%EC%83%89-%ED%8C%8C%EB%9D%BC%EB%A9%94%ED%8A%B8%EB%A6%AD-%EC%84%9C%EC%B9%98-1790-%EC%88%98-%EC%9D%B4%EC%96%B4-%EC%93%B0%EA%B8%B0-2

 

 

#if 1
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <vector>
#include <list>
#include <queue>
#include <unordered_map>
#include <cmath>
#define ll long long
#define ull unsigned long long
using namespace std;

int N, K;
ull low, high, ans;

int getStand(int n)
{
	int ret = 0;
	while (n > 0) n /= 10, ret++;
	return ret;
}

ull getLen(int n)
{
	int len = getStand(n);
	ull ret = 0;
	while (len > 0)
	{
		int tmp = n - pow(10, len - 1) + 1;
		ret += len * tmp;
		n -= tmp;
		len--;
	}
	return (ret);
}
int main(void)
{
	//ios_base::sync_with_stdio(false);
	//cin.tie(nullptr);
	//cout.tie(nullptr);
	freopen("input.txt", "r", stdin);

	cin >> N >> K;
	low = 1;
	high = N;
	ull len = getLen(N);
	if (len < K) {
		cout << -1 << '\n';
		return 0;
	}
	while (low <= high)
	{
		ull mid = (low + high) / 2;
		len = getLen(mid);
		if (len < K) low = mid + 1;
		else ans = mid, high = mid - 1;
	}

	char buf[20];
	int s = sprintf(buf, "%d", ans);
	len = getLen(ans);
	cout<< buf[s - (len - K) - 1] - '0'<<'\n';
}
#endif