力扣-字符串-28 找出字符串中第一个匹配项的下标

思路

kmp算法的练习，实际上来说在构建next数组和使用next数组都用到了前一位字符串的最长相等前后缀

代码

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

            if(needle[j] == haystack[i]) j++;


            if(j == needle.size()){
                return i - j + 1;
            }
        }

        return -1;

    }
};