UVA 644 Immediate Decodability

Immediate Decodability

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 

01
10
0010
0000
9
01
10
010
0000
9

Sample Output 

Set 1 is immediately decodable
Set 2 is not immediately decodable

题目意思很简单，就是判断某一串01代码，是不是另一串的前缀，存在某一串是另一串的前缀的话，就输出

is not immediately decodable

否则输出

Set 1 is immediately decodable

解决这个问题时，我采取的办法是先用qsort将字符串怕序，这样要是有前缀的出现的话，就一定会是相邻的，所以只需要相邻的两两比较，节省很多时间

AC代码：

#include<iostream>
#include<cstring>
#include<cstdio>
#include<cstdlib>
using namespace std;
int Comp (const void *p1 , const void *p2){

    return strcmp((char*)p1,(char*)p2);
}

int main () {

    int num = 1;
    int T=1;
    bool find=false;
    char str[100][50];
    while (scanf("%s",str[0])!=EOF){
        getchar();
        for (int i = 1 ;; i++ ) {
            num++;
            scanf("%s" , str[i]);
            getchar();
            if ( str[i][0] == '9' )
                break;
        }
        qsort(str,num,50,Comp);
        int flag = 0;

        for ( int j = 0 ; j < num - 2  ; j++){
            for ( int k = 0 ; k <( strlen ( str[j] ) <= strlen ( str [j+1]) ? strlen(str[j]) : strlen(str[j+1]) ); k++ ){
                if ( str[j][k] == str[j+1][k] ){
                    flag++;
                }
            }
            if ( flag == ( strlen ( str[j] ) <= strlen ( str [j+1]) ? strlen(str[j]) : strlen(str[j+1])) ){
                find = true ;
                cout << "Set " << T++ << " " << "is not immediately decodable" << endl;
                break;
            }
            flag = 0;

        }

        if ( find == false )
            cout << "Set " << T++ << " " << "is immediately decodable" <<endl;
        for (int i = 0 ; i < 100 ; i++)
            memset(str[i], 0,50);
        num = 1;
        find = false;

    }
        return 0;
    
}