学步园

大意略。

思路：可以发现有循环节，之后在循环节中找最大的即可，可过STL或者哈希表来判重，另一种巧妙的方法是通过Floyd判圈算法来实现。

通过这题熟悉了C++的stringstream和Floyd判圈算法。

哈希表：

#include <iostream>   
#include <sstream>
#include <cstdlib>   
#include <cstdio>   
#include <string>   
#include <cstring>   
#include <cmath>   
#include <vector>   
#include <queue>   
#include <stack>   
#include <algorithm>
#include <cctype>
using namespace std;  

const int maxn = 100003;
const int INF = 0x3f3f3f3f;

typedef long long LL;

int st[maxn];
int rear;

struct Hash
{
	int first[maxn], next[maxn];
	int hash(int s) { return (s & 0x7fffffff) % maxn; }
	void init() { memset(first, -1, sizeof(first)); }

	void insert(int s)
	{
		int h = hash(st[s]);
		next[s] = first[h];
		first[h] = s;
	}

	int find(int s)
	{
		int h = hash(s);
		for(int v = first[h]; v != -1; v = next[v])
		{
			if(s == st[v]) return 1;
		}
		return 0;
	}
};

Hash hash;

int n, k;

void read_case()
{
	hash.init();
	scanf("%d%d", &n, &k);
}

int next(int n, int k)
{
	stringstream ss;
	ss << (LL) k*k;
	string s = ss.str();
	if(s.length() > n) s = s.substr(0, n);
	int ans;
	stringstream ss2(s);
	ss2 >> ans;
	return ans;
}

void solve()
{
	rear = 0;
	read_case();
	int ans = -INF;
	while(!hash.find(k))
	{
		st[rear] = k;
		hash.insert(rear);
		rear++;
		if(ans < k) ans = k;
		k = next(n, k);
	}
	printf("%d\n", ans);
}

int main()
{
	int T;
	scanf("%d", &T);
	while(T--)
	{
		solve();
	}
	return 0;
}

Floyd判圈：

#include <iostream>   
#include <sstream>
#include <cstdlib>   
#include <cstdio>   
#include <string>   
#include <cstring>   
#include <cmath>   
#include <vector>   
#include <queue>   
#include <stack>   
#include <algorithm>
#include <cctype>
using namespace std;  

typedef long long LL;

int n, k;

/*int next(int n, int k)
{
	stringstream ss;
	ss << (LL) k*k;
	string s = ss.str();
	if(s.length() > n) s = s.substr(0, n);
	int ans;
	stringstream ss2(s);
	ss2 >> ans;
	return ans;
}*/

/*int next(int n, int k)
{
	if(!k) return 0;
	LL k2 = (LL)k*k;
	int L = 0;
	while(k2)
	{
		buf[L++] = k2%10;
		k2 /= 10;
	}
	if(n > L) n = L;
	int ans = 0;
	for(int i = 0; i < n; i++) ans = ans*10 + buf[--L];
	return ans;
}*/

int next(int n, int k)
{
	char buf[100], s1[100];
	sprintf(buf, "%lld", (LL)k*k);
	if(n > strlen(buf)) n = strlen(buf);
	strncpy(s1, buf, n), s1[n] = '\0';
	int ans = 0;
	for(int i = 0; i < n; i++) ans = ans*10 + s1[i]-'0';
	return ans;
}

void read_case()
{
	scanf("%d%d", &n, &k);
}

void solve()
{
	read_case();
	int ans = k;
	int count = 0;
	int k1 = k, k2 = k;
	do
	{
		k1 = next(n, k1);
		k2 = next(n, k2); if(k2 > ans) ans = k2;
		k2 = next(n, k2); if(k2 > ans) ans = k2;
		//count++; //算出循环节 
	}while(k1 != k2);
	printf("%d\n", ans); //printf("%d\n", count);
}

int main()
{
	int T;
	scanf("%d", &T);
	while(T--)
	{
		solve();
	}
	return 0;
}