[006] largest_common_substring

[Description] Given two different strings, find the largest successive common substring.

e.g.　　str1[]="qwertyuiopasdfgh";　　str2[]="jhdfgqwertyudfxcv";

　　      LCsubstr[]="qwertyu";

[Thought] show the correspondence result of this two strings from the 2-D array of ‘record[][]‘, and find the longest non-zero diagonal elements. To save the auxiliary space, we can use only 1-D array ‘record[]‘ to implement. 　　O(n^2)　　O(m)

[Implementation] C code:

 1 #include<stdio.h>
 2 #include<stdlib.h>
 3 #include<string.h>
 4
 5 char* LCS(char longer[], char shorter[])
 6 {
 7         int longer_len=strlen(longer);
 8         int shorter_len=strlen(shorter);
 9         int record[shorter_len];
10         int i,j,max_len=0,substr_end=0,substr_begin;
11
12         for(i=0;i<longer_len;i++)
13         {
14                 for(j=shorter_len-1;j>=0;j--)
15                 {
16                         if(longer[i]==shorter[j])
17                         {
18                                 if(0==i || 0==j)
19                                 {
20                                         record[j]=1;
21                                 }
22                                 else
23                                 {
24                                         record[j]=record[j-1]+1;
25                                 }
26                         }
27                         else
28                         {
29                                 record[j]=0;
30                         }
31
32                         if(record[j]>max_len)
33                         {
34                                 max_len=record[j];
35                                 substr_end=j;
36                         }
37                 }
38         }
39
40         substr_begin=substr_end-max_len+1;
41
42 //      char substr[max_len];
43         char* substr=(char*)malloc(max_len);
44         for(i=0;i<max_len;i++)
45         {
46                 substr[i]=shorter[substr_begin+i];
47         }
48         return substr;
49 }
50
51 int main()
52 {
53         char longer[]="asdfghjklqwertyyuio";
54         char shorter[]="zxvsdffghwerxv";
55
56         printf("The longer str:\n\t%s\n",longer);
57         printf("The shorter str:\n\t%s\n",shorter);
58         printf("The lagest common str:\n\t%s\n",LCS(longer,shorter));
59         return 0;
60 }