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