Calgary Collegiate Programming Contest 2008 / UVa 11549 Calculator Conundrum (Floyd判圈算法&更快的優化)

11549 - Calculator Conundrum

Time limit: 6.000 seconds

http://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=460&page=show_problem&problem=2544

Alice got a hold of an old calculator that can display n digits. She was bored enough to come up with the following time waster.

She enters a number k then repeatedly squares it until the result overflows. When the result overflows, only the n most significant digits are displayed on the screen and an error flag appears. Alice can clear the error and continue squaring the displayed number. She got bored by this soon enough, but wondered:

“Given n and k, what is the largest number I can get by wasting time in this manner?”

Program Input

The first line of the input contains an integer t (1 ≤ t ≤ 200), the number of test cases. Each test case contains two integers n (1 ≤ n ≤ 9) and k (0 ≤ k < 10n) where n is the number of digits this calculator can display k is the starting number.

Program Output

For each test case, print the maximum number that Alice can get by repeatedly squaring the starting number as described.

Sample Input & Output

INPUT

2
1 6
2 99
      

OUTPUT

9
99      

首先按題意這麼算，肯定會出現循環。

如何判斷剛好出現循環的位置？——Floyd判圈算法。

完整代碼：

/*0.212s*/

#include <cstdio>
#include <algorithm>
using namespace std;
typedef long long LL;

LL n, k, base;

LL count(LL x)///取前n個數的優化寫法
{
	while (x >= base)
		x /= 10;
	return x;
}

int main(void)
{
	int cas;
	scanf("%d", &cas);
	while (cas--)
	{
		LL ans, k1, k2;
		scanf("%lld%lld", &n, &k);
		base = 1;
		while (n--)
			base *= 10;
		k1 = k2 = k;
		ans = k;
		do
		{
			k1 = count(k1 * k1);
			k2 = count(k2 * k2);ans = max(ans, k2);
			k2 = count(k2 * k2);ans = max(ans, k2);
		}
		while (k1 != k2);
		printf("%lld\n", ans);
	}
}