KMP

#include<bits/stdc++.h>
using namespace std;
const int maxn=1e2;

int _next[maxn];

void initnext(string str){
	int i,j;
	_next[0]=-1;
	int len = str.size();
	for(i=1;i<len;i++){
		j=_next[i-1];
		while(str[j+1]!=str[i]&&j>=0)j=_next[j];
		if(str[i]==str[j+1])_next[i]=j+1;
		else _next[i]=-1;
	}
}

int kmp(string str,string ptr){
	int slen=str.size(),s=0;
	int plen=ptr.size(),p=0;
	initnext(ptr);
	while(s<slen&&p<plen){
		if(str[s]==ptr[p])s++,p++;
		else {
			if(p==0)s++;
			else p=_next[p-1]+1;
		}
	}
	return p==plen?s-plen:-1;

}

int main(){
	string a="aaassdewaaaaswed";
	string b="aaaas";
	string c="wef";
	cout<<kmp(a,b)<<"\n";
	cout<<kmp(a,c)<<"\n";
}

输出

8
-1

版本二

#include<bits/stdc++.h>
using namespace std;
const int maxn = 1e3+3;
char str[maxn],pattern[maxn];
int fail[maxn],cnt;

void getFail(char *p,int plen){
	fail[0]=0;fail[1]=0;
	for(int i=1;i<plen;i++){
		int j=fail[i];
		while(j&&p[i]!=p[j])j=fail[j];
		fail[i+1]=(p[i]==p[j])?j+1:0;
	}
}

int kmp(char *s,char *p){
	int last = -1;
	int slen = strlen(s),plen=strlen(p);
	getFail(p,plen);
	int j =0;
	for(int i=0;i<slen;i++){
		while(j&&s[i]!=p[j])j=fail[j];
		if(s[i]==p[j])j++;
		if(j==plen){
			printf("%d\n",i+1-plen);
			if(i-last>=plen){//判断新的匹配和上一个匹配能否分开
				cnt++;
				last=i;
			}
		}
	}
}

int main(){
	while(~scanf("%s%s",str,pattern)){
		cnt = 0;
		kmp(str,pattern);
		printf("%d\n",cnt); 
	}
}