Immediate Decodability

本文链接：https://blog.youkuaiyun.com/u014733623/article/details/27969849

An encoding of a set of symbols is said to be immediately decodable if no code for one symbol is the prefix of a code for another symbol. We will assume for this problem that all codes are in binary, that no two codes within a set of codes are the same, that each code has at least one bit and no more than ten bits, and that each set has at least two codes and no more than eight.

Examples: Assume an alphabet that has symbols {A, B, C, D}

The following code is immediately decodable:

A:01 B:10 C:0010 D:0000

but this one is not:

A:01 B:10 C:010 D:0000 (Note that A is a prefix of C)

Input

Write a program that accepts as input a series of groups of records from a data file. Each record in a group contains a collection of zeroes and ones representing a binary code for a different symbol. Each group is followed by a single separator record containing a single 9; the separator records are not part of the group. Each group is independent of other groups; the codes in one group are not related to codes in any other group (that is, each group is to be processed independently).

Output

For each group, your program should determine whether the codes in that group are immediately decodable, and should print a single output line giving the group number and stating whether the group is, or is not, immediately decodable.

The Sample Input describes the examples above.

Sample Input

Sample Output

Set 1 is immediately decodable
Set 2 is not immediately decodable

题意：不能让某个字符串是某个其他字符串的前缀，因为这样读一个连续的字符串去解码的时候就会出现歧义。

思路：先将所有字符串按长度排序，先处理长的字符串，每次处理时用哈希判断是否存在这个前缀的字符串，然后将这个字符串的所有前缀记录下来。

AC代码如下：

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<cmath>
using namespace std;
int hash[102410];
struct node
{ char s[100];
  int len;
}str[10000010];
bool cmp(node a,node b)
{ return a.len>b.len;
}
int main()
{ int i,j,k,t=1,len,pos=0;
  bool flag=true;
  while(~scanf("%s",str[++pos].s))
  { if(str[pos].s[0]=='9')
    { flag=true;
      pos--;
      for(i=1;i<=pos;i++)
       str[i].len=strlen(str[i].s);
      sort(str+1,str+1+pos,cmp);
      for(i=1;i<=pos;i++)
      { k=0;len=str[i].len;
        for(j=0;j<len;j++)
         k=k*3+str[i].s[j]-'0'+1;
        if(hash[k]==t)
        { flag=false;
          break;
        }
        k=0;
        for(j=0;j<len;j++)
        { k=k*3+str[i].s[j]-'0'+1;
          hash[k]=t;
        }
      }
      if(flag)
       printf("Set %d is immediately decodable\n",t++);
      else
       printf("Set %d is not immediately decodable\n",t++);
      flag=true;
      pos=0;
    }

  }
}