leetcode -- Implement strStr() -- KMP难理解，但考得不多，知道就好

https://leetcode.com/problems/implement-strstr/

用KMP 算法解最好。很难理解，知道就好。interview不会问到这个程度？

ref：
http://wiki.jikexueyuan.com/project/kmp-algorithm/define.html

参考这个code
http://www.geeksforgeeks.org/searching-for-patterns-set-1-naive-pattern-searching/
暴力法
http://codesays.com/2014/solution-to-implement-strstr-by-leetcode/

class Solution(object):

    def computeLPS(self, pat, M, lps):

        len = 0
        lps[0]
        i = 1
        while i < M:

            if pat[i] == pat[len]:
                len += 1
                lps[i] = len
                i += 1
            else:
                if len != 0:
                    len = lps[len - 1] # recursive
                else:
                    lps[i] = 0
                    i += 1

    def strStr(self, haystack, needle):
        """ :type haystack: str :type needle: str :rtype: int """
        if not needle:
            return 0

        len_pat = len(needle)
        len_txt = len(haystack)
        lps = [0] * len_pat
        self.computeLPS(needle, len_pat, lps)

        i, j = 0, 0

        while i < len_txt:
            if needle[j] == haystack[i]:
                i += 1
                j += 1

            if j == len_pat:
                return i - len_pat
            elif i < len_txt and needle[j] != haystack[i]:
                if j!=0 :
                    j = lps[j - 1]
                else:
                    i +=1
        return -1