KMP字符串匹配算法

#include<iostream>
#include<string>
#include<vector>
using namespace std;
bool matched(string s,int k,int i)
{
    bool result=true;
    int p=0;
    int q=i;
    while(p<k)
    {
        if(s[p]==s[q])
        {
            p++;q--;
        }
        else
        {
            result=false;
            break;
        }
    }
    return result;
}
vector<int> find_next(string p)
{
    vector<int> result;
    result.push_back(0);
    for(int i=1;i<p.length();i++)
    {
        int k=0;
        for(k=i;k>0;k--)
        {
            if(matched(p,k,i))
            {
                result.push_back(k);
                break;
            }
        }
        if(k==0)
            result.push_back(0);
    }
    return result;
}
int main()
{
    string s,p;
    cin>>s>>p;
    
    vector<int> next_index=find_next(p);
    int index_s=0,index_p=0;
    int matched=0;
    while(index_s<s.length() && index_p<p.length())
    {
        if(s[index_s]==p[index_p])
        {
            index_s++;index_p++;matched++;
        }
        else if(matched==0)
        {
            index_s++;
        }
        else
        {
            index_p=matched-next_index[index_p-1];
            matched=index_p-1;
        }
    }
    if(index_p==p.length())
    {
        cout<<index_s-index_p<<endl;
        system("pause");
        return 0;
    }
    else
    {
        cout<<-1<<endl;
        system("pause");
        return 0;
    }
}