模式匹配---KMP算法

//KMP算法的效率主要是去掉了指针回溯，借助于实效函数，可以高效的移动指针到下一个检查位置
#include <string>
#include <iostream>
#include <iomanip>
#include <conio.h>
using namespace std;
void fail(char *src, int *f);
int KmpFind(char *src, char *pat, int fail[]);
int main()
{
char src[100], pat[100];
int f[100],pos;
cout<<"输入原字符串:"<<endl;
cin.getline(src,100);
cout<<"输入子串:"<<endl;
cin.getline(pat,100);
fail(src,f);
pos = KmpFind(src,pat,f);
if(pos == -1) cout<<src<<" 中没有 "<<pat<<" 子串 "<<endl;
else cout<<src<<" 中子串 "<<pat<<" 起始位置为: "<<pos<<endl;
getch();
return 0;
}
void fail(char *src , int *f)//src为原字符串，f[]为失效函数
{
int lengthP = strlen(src);
f[0] = -1;
for(int j = 1; j < lengthP; j++)
{
int i = f[j-1];
while(*(src + j) != *(src + i + 1) && i >= 0) i = f[i];
if(*(src +j) == *(src + i + 1)) f[j] = i + 1;
else f[j] = -1;
}
}

int KmpFind(char *src, char *pat, int fail[])//fail[]为失效函数
{
int posP = 0, posT = 0;
int lengthP = strlen(pat), lengthT = strlen(src);
while(posP < lengthP && posT < lengthT)
{
if(pat[posP] == src[posT])
{
posP++;
posT++;
}
else if(posP == 0)
{
posT++;
}
else posP = fail[posP - 1] + 1;
}
if(posP < lengthP)
return -1;
else return posT - lengthP + 1;
}