28. Implement strStr()(KMP字符串匹配算法)

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Implement strStr().

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

Example 1:

Input: haystack = "hello", needle = "ll"
Output: 2

Example 2:

Input: haystack = "aaaaa", needle = "bba"
Output: -1

暴力解法:如果模式串匹配失败，需要回溯到模式串的起始位置

我们想可以不用回溯到起始位置，如图：
如果能确定A==B 可以直接跳到C跟d比较

问题就转化成了如何求模式串中前缀串于后缀串相等的K个

 class Solution:
     def strStr(self, haystack, needle):
         """
         :type haystack: str
         :type needle: str
         :rtype: int
         """
         if needle =='':
             return 0
         nexts=self.caclNext(needle)
         ans = -1
         i = 0
         j = 0
         while i < len(haystack):
             if(j==-1 or haystack[i] == needle[j]):
                 i += 1
                 j += 1
             else:
                 j = nexts[j]
             if j == len(needle):
                 ans = i - len(needle)
                 break
         return ans

     def caclNext(self, p):
         nexts = [0]*(len(p))
         nexts[0] = -1
         k = -1
         j = 0
         while j < len(p) - 1:
             if k == -1 or p[j] == p[k]:
                 k += 1
                 j += 1
                 nexts[j] = k
             else:
                 k = nexts[k]
         return nexts