Leetcode 28 Implement strStr()

Implement strStr().

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

在一个字符串中查找另外一个字符串是否存在。

暴力 O(n^2):以原字符串的index为指针，每次都检测接下来一段是否与子字符串一样。

class Solution:

    # @param {string} haystack

    # @param {string} needle

    # @return {integer}

    def strStr(self, haystack, needle):

        if needle == '':

            return 0

        for i in range(len(haystack)-len(needle)+1):

            if haystack[i:i+len(needle)] == needle:

                return i

        return -1

KMP算法 O(n): https://en.wikipedia.org/wiki/Knuth–Morris–Pratt_algorithm

http://blog.csdn.net/v_july_v/article/details/7041827

失配时，模式串向右移动的位数为：已匹配字符数 - 失配字符的上一位字符所对应的最大长度值

def str_str(string, substring)return 0 if substring == ''

   

   # create failure function table

   pos = 2

   cnd = 0

   failure_table = [-1, 0]

   while pos < substring.length

     if substring[pos - 1] == substring[cnd]

       failure_table[pos] = cnd + 1

       pos += 1

       cnd += 1

     elsif cnd > 0

       cnd = failure_table[cnd]

     else

       failure_table[pos] = 0

       pos += 1

     end

   end



   m = i = 0

   while m + i < string.length

     if substring[i] == string[m + i]

       i += 1

       return m if i == substring.length

     else

       m = m + i - failure_table[i]

       i = failure_table[i] if i > 0

     end

   end

   -1

end