KMP算法 字符串匹配

用kmp算法实现字符串匹配，kmp算法的介绍

int strStr(string haystack, string needle) {        if (needle.size() == 0)            return 0;        vector<int> next(needle.size(), 0);        for (int i = 2; i < next.size(); ++i)        {            int j = next[i - 1];            while (j > 0 && needle[j] != needle[i - 1])                j = next[j];            if (needle[j] == needle[i-1])                next[i] = j + 1;        }        int k = 0;        while (k < haystack.size())        {            bool flag = true;            for (int i = 0; i < needle.size(); ++i)            {                if (k + i >= haystack.size())                    return -1;                if (needle[i] != haystack[k + i])                {                    if (i == 0)                        ++k;                    else                        k += i - next[i];                    flag = false;                    break;                }            }            if (flag)                return k;        }        return -1;    }