KMP模板

void getnext(string s){//获得next数组
	int i,j;
	i=0,j=-1;
	nxt[0]=-1;
	while(i<s.size()){
		if(j==-1||s[i]==s[j]){
			nxt[++i]=++j;
		}     
		else
			j=nxt[j];
	}  
}

或者

void get_Next(string s){
	int j=0;	
	nxt[0]=-1;
	for(int i=1;i<s.size();i++){	
		while(j>0&&s[i]!=s[j])j nxt[j-1];	
		if(s[i]==s[j]){
			j++;
		}		
		nxt[i]=j;	
	}
}

int kmp(string s, string t){//kmp 匹配
	if(t.size()==0)	return 0;
	get_Next(t, next);
	int j = 0;
	for(int i=0;i<s.size();i++){
		while(j>0&&s[i]!= t[j])	j=next[j-1];	
		if(s[i]==t[j])	j++;
		if(j==t.size())	return i-t.size()+1;
	}
	return -1;
}

模板二

#include <bits/stdc++.h>
using namespace std;

const int N = 1e6 + 5;

char a[N], b[N];
int nxt[N];
int T, n, m;

int solve(char *a, char *b, int n, int m) {
	for (int i = 2; i <= n; i++) {
		int k = nxt[i - 1];
		while (k && a[i] != a[k + 1]) k = nxt[k];
		if (a[i] == a[k + 1]) ++k;
		nxt[i] = k;
	}
	int  k = 0;
	for (int i = 1; i <= m; i++) {
		while (k && b[i] != a[k + 1]) k = nxt[k];
		if (b[i] == a[k + 1]) ++k;
		if (k == n) return i-n+1;
	}
	return -1;
}

int main() {
	scanf("%d", &T);
	while (T--) {
		scanf("%s%s", a + 1, b + 1);
		n = strlen(a + 1); m = strlen(b + 1);
		cout<<solve(a, b, n, m)<<endl;
		//b里面找a 
	}
	return 0;
}