为什么注释的那样写得到的结果是错的

#include<iostream>
#include<string>
using namespace std;
int index_BF(string s,string t,int pos)
{
    int j=0,i=pos;
    while(i<s.length()&&j<t.length()) {
        if(s[i]==t[j]) {
            i++;
            j++;
        }
        else {
            i=i-j+1;
            j=0;
        }
    }
    if(j==t.length()) return i-j;
    return -1;
}
void getNext(string t,int next[]) {
    int j=-1,i=0;
    next[0]=-1;
    while(i<t.length()-1) {
        if(j==-1||t[i]==t[j]) {
            i++;
            j++;
            next[i]=j;
        } else {
            j=next[j];
        }
    }
}
void GetNext(string p,int next[])
{
    int pLen = p.length();
    next[0] = -1;
    int k = -1;
    int j = 0;
    while (j < pLen - 1)
    {
        if (k == -1 || p[j] == p[k])
        {
            ++k;
            ++j;
            next[j] = k;
        }
        else
        {
            k = next[k];
        }
    }
}
int KmpSearch(string s, string p)
{
    int next[100];
    int i = 0;
    int j = 0;
    int sLen = s.length();
    int pLen = p.length();
    GetNext(p,next);
    while (i < sLen && j < pLen)
    {
        if (j == -1 || s[i] == p[j])
        {
            i++;
            j++;
        }
        else
        {
            j = next[j];
        }
    }
        if (j == pLen)
        return i - j;
    else
        return -1;
}
int index_kmp(string s,string t,int pos)
{
    int next[100];
    int j=0,i=pos;
    GetNext(t,next);
    int sLen = s.length();
    int tLen = t.length();
    for(int x=0; x<tLen; x++)
        cout<<next[x]<<" ";
    cout << endl;
    while(i<s.length()&&j<tLen)
    //while(i<s.length()&&j<t.length())
    {
        if(j==-1||s[i]==t[j]) {
            i++;
            j++;
        } else {j=next[j];}
    }
    //cout << t.length() << endl;
    if(j==t.length()) return i-j;
    return -1;
}
int main()
{
    string s,t;
    while(cin>>s>>t) {
        //cout<<"BF:"<<index_BF(s,t,0)<<endl;
        cout<<"KMP:\n"<<index_kmp(s,t,0)<<endl;
        cout << KmpSearch(s,t) << endl;
    }
    return 0;
}

想不明白，求解

这两种按你写的是等价的，有报错吗