KMP

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn = 1e6 + 10;
int nxt[maxn];
char a[maxn], b[maxn];

void getNext(char p[]) {
	int i = 0, k = -1, len = strlen(p);
	nxt[i] = -1;
	while(i < len) {
		if(k == -1 || p[i] == p[k]) {
			if(p[++i] != p[++k]) nxt[i] = k;
			else nxt[i] = nxt[k];	// 注意 
		} else k = nxt[k];
	}
}
// 匹配次数 
int kmp_match(char t[], char p[]) {
	int i = 0, j = 0, cnt = 0;
	int n = strlen(t), m = strlen(p);
	while(i <= n - 1) {
		if(j < 0 || t[i] == p[j]) i++, j++;
		else j = nxt[j];
		if(j == m) cnt++, j = nxt[j];
	}
	return cnt;
}
// 匹配位置 
int kmp_search(char t[], char p[]) {
	int i = 0, j = 0;
	int n = strlen(t), m = strlen(p);
	while(i < n && j < m) {
		if(j < 0 || t[i] == p[j]) i++, j++;
		else j = nxt[j];
	}
	if(j == m) return i - j + 1;
	else return -1;
}

int main() {
	int n, m, T;
	scanf("%d", &T);
	while(T--) {
		scanf("%d%d", &n, &m);
		scanf("%s%s", a, b);
		getNext(b);
		int ans = kmp_search(a, b);
		printf("%d\n", ans);
	}
}