# 谁有KMP算法的C语言实现啊-问答-阿里云开发者社区-阿里云

## 谁有KMP算法的C语言实现啊

• 青衫无名
2019-07-17 22:55:56
其基本思想是：每当匹配过程中出现字符串比较不等时，不需回溯i指针，而是利用已经得到的“部分匹配”结果将模式向右“滑动”尽可能远的一段距离后，继续进行比较。

#include <stdio.h>
#include <string.h>
int index_KMP(char *s,char *t,int pos);
void get_next(char *t,int *);

char s[10]="abcacbcba";
char t[4]="bca";
int next[4];
int pos=0;

int main()
{
int n;
get_next(t,next);
n=index_KMP(s,t,pos);
printf("%d",n);
return 0;
}

int index_KMP(char *s,char *t,int pos)
{
int i=pos,j=1;
while (i<=(int)strlen(s)&&j<=(int)strlen(t))
{
if (j==0||s[i]==t[j-1])
{
i++;
j++;
}
else j=next[j];
}
if (j>(int)strlen(t))
return i-strlen(t)+1;
else
return 0;
}

void get_next(char *t,int *next)
{

int i=1,j=0;
next[0]=next[1]=0;
while (i<(int)strlen(t))
{
if (j==0||t[i]==t[j])
{
i++;
j++;
next[i]=j;
}
else j=next[j];
}

}
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• 一键天涯
2019-07-17 22:55:56
#include<stdio.h>
#include<malloc.h>
#include<string.h>
#include<assert.h>
#include<stdlib.h>
void getNext(const char * t,int * Next)//get the Next array
{
int k=-1;
int j=0;
int size=strlen(t);
Next[0]=-1;
while(j<size)
{
if(k==-1||t[j]==t[k])//if k==-1 there are two conditions
//one is this is the first time entering the loop
{ //if t[j]==t[k] get the next[j+1] directly
k++;//the other is the end of the iteration cos k==-1;
j++;
Next[j]=k;//whatever Next[j]=k
}
else
k=Next[k];
}
}
int myStrstr(const char * Dest,const char *subStr)//find the starting position of the sub
{ //in the Dest through KMP
int destSize=strlen(Dest);
int subSize=strlen(subStr);
int i,j;
int * Next=(int *)(malloc(sizeof(int)*subSize));
i=j=0;
assert((Dest!=NULL)&&(subStr!=NULL));
getNext(subStr,Next);
while(i<destSize&&j<subSize)
{
if(j==-1||Dest[i]==subStr[j])//if j==-1 the main string need match the next elements
{ //and the subString begin from the beginning
i++; //if Dest[i]==subStr[j] both string need shift to the
j++; // next elements
}
else
j=Next[j]; //if match fail,glide back to Next[j]
}
if(j==subSize)return i-j;
return -1;
}
int main()
{
char *temp,*sub,* Dest;//to store the substring to be matched
int ch;
unsigned int templen,length=20*sizeof(char);
unsigned int mlength=20*sizeof(char);//the original length of the memory
sub=(char*)malloc(length);
Dest=(char*)malloc(length);
if(sub==NULL||Dest==NULL)//allocation failure
{
printf("memory allocate failure\n");
exit(0);
}
temp=sub;
{
if((temp-sub)/sizeof(char)==length)//if running out of the memory
{
templen=length;
sub=realloc(sub,length+=20*sizeof(char));
if(sub==NULL)
{
printf("sub memory allocate failure\n");
exit(0);
}
temp=sub+templen*sizeof(char);//reset the temp cos the sub may change its value
}
*temp++=ch;
}
*temp='\0';
temp=Dest;
{
if((temp-Dest)/sizeof(char)==mlength)
{
templen=mlength;
Dest=realloc(Dest,mlength+=20*sizeof(char));//if running out of the memory
if(Dest==NULL)
{
printf("sub memory allocate failure\n");
exit(0);
}
temp=Dest+templen*sizeof(char);//reset the temp cos the Dest may change its value
}
*temp++=ch;
}
*temp='\0';
printf("the starting position is :%d\n",myStrstr(Dest,sub));//get the starting position
free(Dest);
free(sub);
return 0;
}
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