KMP算法next数组的求法：

#include <iostream>
#include <vector>
#include <cstring>
using namespace std;

void MakeNext(const string &P, vector<int> &next)
{
    int q,k;
    int m = P.size();
    next[0] = -1;
    for (q = 1, k = 0; q < m; ++q)
    {
        while (k > 0 && P[q] != P[k])
            k = next[k-1];
        if (P[q] == P[k])
            k++;
        next[q+1] = k;
    }
}

int main()
{
    vector<int> next(20,0);
    string P = "ababc";
    MakeNext(P,next);
    for (int i = 0; i < P.size(); i++)
        cout<<"next["<<i<<"]:"<<next[i]<<"  ";
    cout << endl;
    return 0;
}
/*
运行结果：-1 0 0 1 2
*/