28. Implement strStr()(KMP)

1刷
KMP经典算法，主要是了解KMP基本原理， next数组怎样求！

class Solution {public:    int next[500001];    void makenext(string s){        int n = s.size();        next[0] = 0;        int k = 0;        for(int i = 1; i < n; ++ i){            while(k > 0 && s[k] != s[i])                k = next[k - 1];            if(s[k] == s[i]) k++;            next[i] = k;        }    }    int strStr(string haystack, string needle) {        int n = haystack.size();        int m = needle.size();        if(m == 0) return 0;        int num = -1;        makenext(needle);        for(int i = 0, j = 0; i < n; ++ i){            while(j > 0 && haystack[i] != needle[j])                j = next[j - 1];            if(haystack[i] == needle[j])                j++;            if(j == m){                num = i - m + 1;                break;            }        }        return num;    }};

2刷
没啥好说的，就是kmp，要记住next数组是怎样求的，其实next的求法跟kmp匹配时候基本大同小异，就是容易忘记，背住那两幅next的图就ok比较好理解。
注意小细节ps
int n = needle.length();
if(n == 0) return 0

注意这个就ok，可以3刷

class Solution {public:    vector<int>next;    void makenext(string s){        int n = s.length();        for(int i = 0; i < n; ++ i) next.push_back(0);        for(int i = 0, j = 1; j < n; ++ j){            while(i > 0 && s[i] != s[j]) i = next[i - 1];            if(s[i] == s[j]) i++;            next[j] = i;        }    }    int index = -1;    int strStr(string haystack, string needle) {        int m = haystack.length();        int n = needle.length();        if(n == 0) return 0;        makenext(needle);        for(int i = 0, j = 0; i < m; ++ i){            while(j > 0 && haystack[i] != needle[j]) j = next[j - 1];            if(haystack[i] == needle[j]) j++;            if(j == n){                index = i - n + 1;                break;            }        }        return index;    }};