Kmp算法

/*
时间复杂度O(a.length() + b.length())
最小覆盖字串(len - next[len - 1] - 1)
单串模式匹配
*/

void get_next(char* s)
{
    next[0] = -1;

    for (int i = 1, j = -1; i < strlen(s); i++)
    {
        while (j >= 0 && s[i] != s[j + 1])
        {
            j = next[j];
        }

        if (s[i] == s[j + 1])
        {
            j++;
        }

        next[i] = j;
    }
}

int kmp(char* a, char* b)
{
    int res = 0, len = strlen(b);
    get_next(b);

    for (int i = 0, j = -1; i < strlen(a); i++)
    {
        while (j >= 0 && a[i] != b[j + 1])
        {
            j = next[j];
        }

        if (a[i] == b[j + 1])
        {
            j++;
        }

        if (j == len - 1)
        {
            res++;
            j = next[j];	//计算有多少个
        }
    }

    return res;
}