KMP算法的两种模板

个人感觉这种比较好理解
讲解的很好的一篇文章KMP算法详解
例题leetcode

class Solution {
     
public:
    int strStr(string haystack, string needle) {
     
        if(!needle.size()) return 0;
        int szn = needle.size(),szh = haystack.size();
        vector<int> nex;
        nex.push_back(0);
        int pos = 0;
        for(int i = 1;i<szn;)
        {
     
            if(needle[i] == needle[pos])pos++,i++,nex.push_back(pos);
            else if(pos) pos = nex[pos - 1];
            else nex.push_back(0),i++;
        }

        pos = 0;
        for(int i = 0;i<szh;)
        {
     
            if(haystack[i] == needle[pos])i++,pos++;
            else if(pos)pos = nex[pos - 1];
            else i++;

            if(pos == szn)return i - pos;
        }

        return -1;
    }
};

加上0这个位置上的哨兵

class Solution {
     
public:
    int strStr(string s, string p) {
     
        int n = s.size(), m = p.size();
        if(m == 0) return 0;
        //设置哨兵
        s.insert(s.begin(),' ');
        p.insert(p.begin(),' ');
        vector<int> next(m + 1);
        //预处理next数组
        for(int i = 2, j = 0; i <= m; i++){
     
            while(j and p[i] != p[j + 1]) j = next[j];
            if(p[i] == p[j + 1]) j++;
            next[i] = j;
        }
        //匹配过程
        for(int i = 1, j = 0; i <= n; i++){
     
            while(j and s[i] != p[j + 1]) j = next[j];
            if(s[i] == p[j + 1]) j++;
            if(j == m) return i - m;
        }
        return -1;
    }
};