杭电1200

题目描述：

Mo and Larry have devised a way of encrypting messages. They first decide secretly on the number of columns and write the message (letters only) down the columns, padding with extra random letters so as to make a rectangular array of letters. For example, if the message is “There’s no place like home on a snowy night” and there are five columns, Mo would write down

t o i o y
h p k n n
e l e a i
r a h s g
e c o n h
s e m o t
n l e w x


Note that Mo includes only letters and writes them all in lower case. In this example, Mo used the character ‘x’ to pad the message out to make a rectangle, although he could have used any letter.

Mo then sends the message to Larry by writing the letters in each row, alternating left-to-right and right-to-left. So, the above would be encrypted as

toioynnkpheleaigshareconhtomesnlewx

Your job is to recover for Larry the original message (along with any extra padding letters) from the encrypted one.

太长的题目，大意就是密码学的棋盘加密吧，直接写了一串代码：

using System;namespace a1{class Program{public static void Main(string[] args){int n;string str=string.Empty;while((str=Console.ReadLine())!=null){n=Convert.ToInt32(str);if(n==0) break;string s=Console.ReadLine();char[] ch=s.ToCharArray();int l=s.Length/n;//多少行int flag=0;for(int i=0;i<n;i++)for(int j=0;j<l;j++){if(flag==0){Console.Write(ch[i+j*n]);flag++;}else{Console.Write(ch[j*n+n-i-1]);flag--;}}Console.WriteLine();}}}}

居然WA，然后不知道自己错在哪里，所以自己造了一个测试：4 iyrhhyoloumahuvveeca，应该输出iloveyouverymuchhaha，然而错了，于是改代码，AC：

using System;namespace a1{class Program{public static void Main(string[] args){int n;string str=string.Empty;while((str=Console.ReadLine())!=null){n=Convert.ToInt32(str);if(n==0) break;string s=Console.ReadLine();char[] ch=s.ToCharArray();int l=s.Length/n;//多少行for(int i=1;i<l;i+=2)for(int j=0;j<n/2;j++){char t;t=ch[i*n+j];ch[i*n+j]=ch[i*n+n-1-j];ch[i*n+n-1-j]=t;}for(int i=0;i<n;i++)for(int j=0;j<l;j++)Console.Write(ch[i+j*n]);Console.WriteLine();}}}}