String mode matching algorithm

This is common and frequently used, and is also the subject of my exam yesterday.

Matching strings can only be compared from the first character.

Int stringindex (const char * pstr, const char * psubstr, int NPOs) <br/>{< br/> int nlen = strlen (pstr ); <br/> int nsublen = strlen (psubstr); <br/> int I = NPOs; <br/> Int J = 0; <br/> while (I <nlen & J <nsublen) <br/> {<br/> If (pstr [I] = psubstr [J]) <br/>{< br/> I ++; j ++; // continue comparison <br/>}< br/> else <br/>{< br/> I = I-j + 2; j = 1; // rollback, rematch <br/>}< br/> If (j = nsublen) <br/>{< br/> return I-j; <br/>}< br/> else <br/> {<br/> return-1; <br/>}</P> <p> int _ tmain (INT argc, _ tchar * argv []) <br/>{< br/> int nret = stringindex ("abcdefghijk", "ijk", 0); <br/> printf ("% d", nret ); <br/> getchar (); <br/> return 0; <br/>}

 

This algorithm can easily match the position of the child string in the parent string.

 

However, a lot of rollback operations are unnecessary. If the parent string is abcabcd, check the child string ABCD first from a and find that the after the first C is incorrect, it is obviously unnecessary to start from the first B of the parent string. In this case, the string can be matched directly from a after the first C.

 

This improved algorithm is the time complexity of this algorithm discovered by two cow workers is O (m + n ).

 

Although this algorithm seems not difficult, it is not easy to implement. After reading the book, these are all textbooks on data structures and algorithms.

I read n articles, that is, I did not write code independently.

When writing, it is really hard to write.