kmp算法

根据《算法导论》上的伪代码修改，书上的字符串索引从1开始，需要修改为从0开始

class Solution {
public:
    int strStr(string haystack, string needle) {
        int sz1 = haystack.size(), sz2 = needle.size();
        if (sz2 == 0) {
            return 0;
        }
        if (sz1 == 0) {
            return -1;
        }
        int q = -1;
        vector<int>trans = _helper(needle);
        for (int i = 0; i < sz1; ++i) {
            while (q > -1 && haystack[i] != needle[q+1]) {
                q = trans[q];
            }
            if (haystack[i] == needle[q+1]) {
                ++q;
            }
            if (q == sz2-1) {
                return i - sz2+1;
            }
        }
        return -1;
    }
private:
    vector<int> _helper(string needle) {
        int sz = needle.size();
        vector<int>res(sz, 0);
        int k = -1;
        res[0] = -1;
        //q表示状态
        for (int q = 1; q < sz; ++q) {
            while (k > -1 && needle[k+1] != needle[q]) {
                k = res[k];
            }
            if (needle[q] == needle[k+1]) {
                ++k;
            }
            res[q] = k;
        }
        return res;
    }
};