力扣28. 找出字符串中第一个匹配项的下标

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

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