Implement strStr()

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这道题就是求一个字符串是否是另一个字符串的子字符串，所以第一种方法首先想到的是KMP算法，其次用暴力破解，还有用素数代表字符，然后根据进位来计算相关和的方法，

但是容易发生溢出，所以就需要采用，大数值或者用KMP算法，这里python的数值好像溢出了，所以就采用KMP，看了两天，里面难点是next数组的求法，具体求法参见july的博客

答案：

class Solution(object):
    def strStr(self, haystack, needle):
        """
        :type haystack: str
        :type needle: str
        :rtype: int
        """
        if not needle:
            return 0
        if not haystack and needle:
            return -1
        haystackLength = len(haystack)
        next = self.getNextArr(needle)
        needleLength = len(needle)
        j = 0
        k = 0
        while j < haystackLength  and k < needleLength:
            if k == -1 or haystack[j] == needle[k]:
                j += 1
                k += 1
            else:
                k = next[k]
        if k == needleLength:
            return j - k
        else:
            return -1


    def getNextArr(self,s):
        arrLength = len(s)
        k = -1
        j = 0
        next = [ i for i in range(arrLength)]
        next[0] = -1
        while j < arrLength - 1:
            if k == -1 or s[k] == s[j]:
                k += 1
                j += 1
                next[j] = k
            else:
                k = next[k]
        return next