28. 找出字符串中第一个匹配项的下标

题目

KMP典中典，可惜我不会。

卡哥思路里的讲解也似懂非懂，跟着卡哥代码敲了一遍

class Solution {
public:
    void getNext(int *next, const string &s)
    {
        int j = -1;
        next[0] = j;
        for (int i = 1; i < s.size(); ++i)
        {
            while (j >= 0 && s[i] != s[j + 1])
                j = next[j];
            if (s[i] == s[j + 1])
                j++;
            next[i] = j;
        }
    }

    int strStr(string haystack, string needle) {
        if (needle.size() == 0)
            return 0;
        vector<int> next(needle.size());
        getNext(&next[0], needle);
        int j = -1;
        for (int i = 0; i < haystack.size(); ++i)
        {
            while (j >= 0 && haystack[i] != needle[j + 1])
                j = next[j];
            if (haystack[i] == needle[j + 1])
                j++;
            if (j == (needle.size() - 1))
                return (i - needle.size() + 1);
        }
        return -1;
    }
};