字符串查找算法之（一）KMP算法

#include <string.h>
#include <iostream>

using namespace std;
// init the prefix array. when comparing, if text[i] != pattern[j],
// then pattern[prefix[j]] should be next check point of pattern against text[i].
//
// say y = prefix[j], y stands for the biggest length of the word pattern[0..(y-1)]
// that makes (pattern[0..(y-1)] == pattern[(j-y+1)..j]).
// In another word, y stand for the next comparation point of pattern.
// (y==0) means no such word, the next comparation should shart from pattern[0].
// Note: it's possible that 0<y<(i-y)<j and 0<(i-y)<y<j.
//
// When comparing, if (text[i] != pattern[j]) then j = prefix[j-1],
// then compare again with text[i] until (j==0)
//
void InitPrefix(const char *pattern, int prefix[])
{
int len = strlen(pattern);
prefix[0] = 0;
for (int i = 1; i < len; i++)
{
int k = prefix[i - 1];
while ((k > 0) && (pattern[i] != pattern[k]))
{
k = prefix[k - 1];
}
if (pattern[i] == pattern[k])
{
prefix[i] = k + 1;
}
else
{
prefix[i]=0;
}
}
return;
}

int StrStr(const char* text, const char* pattern)
{
if (!text || !pattern || pattern[0] == '/0' || text[0] == '/0')
{
return -1;
}

int lenText = strlen(text);
int lenPattern = strlen(pattern);

if (lenText < lenPattern)
{
return -1;
}

int *prefix = new int[lenPattern + 1];
InitPrefix(pattern, prefix); //the prefix array of pattern

int index = -1; // the index to be returned.
int i = 0; // the compare index in text.
int j = 0; // the compare index in pattern
while ((text[i] != '/0') && ((lenText - i) >= (lenPattern - j)))
{
if (pattern[j] == text[i])
{
//reach the end of pattern, find match! return.
if (pattern[j+1] == '/0')
{
index = i - lenPattern + 1;
break;
}
// compare next char in pattern and text.
j++;
i++;
}
else
{
if (j == 0)
{
// pattern[0] != text[i], then skip to text[i+1].
i++;
}
else
{
//pattern[j]!= text[i], the next check point should be pattern[prefix[j-1]]
j = prefix[j-1];
}
}
}
delete []prefix;
prefix=0;
return index;
}