LeetCode Implement strStr()

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Update (2014-11-02):
The signature of the function had been updated to return the index instead of the pointer. If you still see your function signature returns a char * or String, please click the reload button  to reset your code definition.

题意：找到第一次匹配的下标。

思路：KMP模板题。

class Solution {public:    void getNext(char *patten, vector<int> &next) {        int i = 0, j = -1;        int len = strlen(patten);        next.resize(len+1);        next[0] = -1;        while (i < len) {            if (j == -1 || patten[i] == patten[j]) {                i++, j++;                next[i] = j;            } else j = next[j];        }    }    int strStr(char *haystack, char *needle) {        if (haystack == NULL || needle == NULL)             return -1;        int i = 0, j = 0;        int lena = strlen(haystack);        int lenb = strlen(needle);        vector<int> next;        getNext(needle, next);        while (i < lena && j < lenb) {            if (j == -1 || haystack[i] == needle[j])                i++, j++;            else j = next[j];                    }        if (j == lenb)             return i - j;        else return -1;    }};