推理过程见详解 .

#include <bits/stdc++.h>
#define LL long long
#define Pi acos(-1.0)
#define INF 2147483646
#define eps 1e-9
#define MS 100009
#define mss 17
using namespace std;
// Notice the data size

int n,m,k;
int p[MS],nex[MS];
char s[MS],ts[MS]; 

void get_next(char ts[],int len){
	int j=0,k=-1;
	nex[0] = -1;
	while(j<len-1){
		if(k==-1||ts[j] == ts[k]){
			// 当 ts[j+1] == ts[nex[j+1]] 时要跳过 
			if(ts[++j] == ts[++k])
				nex[j] = nex[k];
			else 
				nex[j] = k;
		}
		else k = nex[k];
	}
}

int kmp(char s[],char ts[],int h1,int h2){
	int i,j;
	i = j = 0;
	while(i<h1 && j<h2){
		if(s[i] == ts[j] || j == -1){
			i++ ,j++;
		}
		else j = nex[j];
	}
	if(j == h2) return i-j; // 返回从 0 开始的主串下标 
	else return -1;
}

int main() {
	cin >> s;
	cin >> ts;
	get_next(ts,strlen(ts));
	int ans = kmp(s,ts,strlen(s),strlen(ts));
	printf("%d\n",ans);
	
	return 0;
}

/*
input : 0123456789
		567
		
output: 5 
*/