KMP算法

#include <bits/stdc++.h>
using namespace std;
const int N=10005;
int next[1005];

//优化过后的next 数组求法

void GetNextval(string p)
{
    int pLen = p.length();
    next[0] = -1;
    int k = -1;
    int j = 0;
    while (j < pLen - 1)
    {
        if (k == -1 || p[j] == p[k])
        {
            ++j;
            ++k;
            if (p[j] != p[k])
                next[j] = k;   //之前只有这一行
            else
                next[j] = next[k];
        }
        else
        {
            k = next[k];
        }
    }
}
int kmp(string s,string p)
{
    int i = 0;
    int j = 0;
    int sLen = s.length();
    int pLen = p.length();
    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 main()
{
   string s1,s2;
   cin>>s1>>s2;
   GetNextval(s2);
   cout<<kmp(s1,s2)<<endl;
}