Educational Codeforces Round 81 (Rated for Div. 2)

Educational Codeforces Round 81 (Rated for Div. 2)

http://codeforces.com/contest/1295

A. Display The Number

思路：首先位数越多越好，那么直接不断的填1，但是只剩3段可以点亮时在首位填个7比较好

#include
using namespace std;
int t,n;
string s;
int main()
{
	scanf("%d",&t);
	while(t--)
	{
		scanf("%d",&n);
		s = "";
		while(n >= 2 && n != 3)
			s+='1',n-=2;
		if(n == 3)
			s = '7' + s;
		cout << s << endl;
	}
	return 0;
}

B. Infinite Prefixes

思路：最终要满足$iy+a=x$,其中y为一个周期的贡献，a为周期中可以得到的贡献，x已知，枚举a看i是否存在即可

#include
#define MAXN 100005
using namespace std;
int t,n,x;
char s[MAXN];
int main()
{
	scanf("%d",&t);
	while(t--)
	{
		scanf("%d%d%s",&n,&x,s);
		int y = 0;
		for(int i = 0;i < n;++i)
		{
			if(s[i] == '0')
				++y;
			else
				--y;
			}
		int ans = 0;
		int tmp = 0;
		bool flag = false;
		for(int i = 0;i < n;++i)
		{
			if(x - tmp == 0)
				flag = true;
			if(y!= 0 && (x-tmp)%y == 0 && (x-tmp)/y >= 0)
				++ans;
			if(s[i] == '0')
				++tmp;
			else
				--tmp;
		}
		if(y == 0 && flag)
			printf("-1\n");
		else
			printf("%d\n",ans);
	}
	return 0;
}

C. Obtain The String

思路：首先贪心，s中字符每次拿越多越好，所以就枚举t中的字符ch看s此时是否可以拿出字符和ch匹配（一轮中拿出的字符是s的子序列），至于是否可以拿出字符和ch匹配是一个套路，先把s中的每个字符出现的下标记录下来即可

#include
#define MAXN 100005
using namespace std;
char s[MAXN],t[MAXN];
vector<int> ve[26];
int T;
int main()
{
	scanf("%d",&T);
	while(T--)
	{
		scanf("%s%s",s,t);
		for(int i = 0;i < 26;++i)
			ve[i].clear();
		int len1 = strlen(s);
		for(int i = 0;i < len1;++i)
			ve[s[i]-'a'].push_back(i);
		int ans = 1;
		int len2 = strlen(t);
		int tmp = 0;
		bool flag = true;
		for(int i = 0;i < len2;++i)
		{
			int ch = t[i]-'a';
			if(ve[ch].size() == 0)
			{
				flag = false;
				break;
			}
			int id = lower_bound(ve[ch].begin(),ve[ch].end(),tmp)-ve[ch].begin();
			if(id != ve[ch].size())
				tmp = ve[ch][id]+1;
			else
				++ans,tmp = 0,--i;
		}
		if(flag)
			printf("%d\n",ans);
		else
			printf("-1\n");
	}
	return 0;
}

D. Same GCDs

见https://blog.csdn.net/xing_mo/article/details/104111794

E. Permutation Separation

见https://blog.csdn.net/xing_mo/article/details/104117702