AtCoder Beginner Contest 336 C - Even Digits题解及做题分享(1)

距离创建这个博客的时间也不久了，到今天我才终于在洛谷写下我的第一篇文章，分享我的寒假训练经历。

本题来自 AtCoder Beginner Contest 336 C - Even Digits。提交三次wa 1，tle 1, AC！！！

ok,话不多说，原题如下：

原题链接

C - Even Digits

Time Limit: 2 sec / Memory Limit: 1024 MB

Score:
300 points

Problem Statement

A non-negative integer
n is called a good integer when it satisfies the following condition:

All digits in the decimal notation of
n are even numbers (
0,
2,
4,
6, and
8).
For example,
0,
68, and
2024 are good integers.

You are given an integer
N. Find the
N-th smallest good integer.

Constraints

1≤N≤10
12

N is an integer.

Sample Input 1

8

Sample Output 1

24

The good integers in ascending order are
0,2,4,6,8,20,22,24,26,28,….
The eighth smallest is
24, which should be printed.

Sample Input 2

133

Sample Output 2

2024

Sample Input 3

31415926535

Sample Output 3

2006628868244228

本人一开始思路为尝试打表，但一算时间复杂度发现是我想得太简单了，于是开始找规律，通过此题样例可以发现数据与五进制有什么不可告人的关系，但苦于能力有限并没有证明出来，但是通过样例不难直接发现规律。

代码如下：

#include<bits/stdc++.h>
using namespace std;
#define int long long
signed main() {
	int n;
	scanf("%lld", &n);
	n--;
	int ans = 0, l = 1;
	while (n > 0) {
		int m = n % 5;
		ans += l * 2 * m;
		n /= 5;
		l *= 10;
	}
	cout << ans << endl;
	return 0;
}