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;
    }
};