[leetcode]Implement strStr() @ Python

原题地址:http://oj.leetcode.com/problems/implement-strstr/

题意:实现字符串匹配函数,并返回一个指针,这个指针指向原字符串中第一次出现待匹配字符串的位置。如:haystack='aabbaa'; needle='bb'。如果使用python实现,则最后返回的应该是一个字符串,即:'bbaa'。

解题思路:这道题我是使用KMP算法写的,到现在KMP算法都不是很懂,只是按照《算法导论》上的伪代码机械的实现了一遍。

代码:

class Solution:
    # @param haystack, a string
    # @param needle, a string
    # @return a string or None
    # @KMP algorithms
    def ComputePrefixFunction(self, needle):
        Pi = [0 for i in range(len(needle))]
        m = len(needle)
        Pi[0] = 0
        k = 0
        for q in range(1, m):
            while k > 0 and needle[k] != needle[q]:
                k = Pi[k - 1]
            if needle[k] == needle[q]:
                k = k + 1
            Pi[q] = k
        return Pi
    
    def strStr(self, haystack, needle):
        n = len(haystack)
        m = len(needle)
        if m == 0:
            return haystack
        Pi = self.ComputePrefixFunction(needle)
        q = 0
        for i in range(0, n):
            while q > 0 and needle[q] != haystack[i]:
                q = Pi[q - 1]
            if needle[q] == haystack[i]:
                q = q + 1
            if q == m:
                return haystack[i - m + 1:]
        return None

暴力匹配的算法,可以ac:

class Solution:
    # @param haystack, a string
    # @param needle, a string
    # @return a string or None
    def strStr(self, haystack, needle):
        if len(haystack) < len(needle): return None
        i = 0
        while i < len(haystack)-len(needle)+1:
            j = 0; k = i
            while j < len(needle):
                if haystack[k] == needle[j]:
                    j+=1; k+=1
                else:
                    break
            if j == len(needle):
                break
            else:
                i+=1
        if i == len(haystack)-len(needle)+1:
            return None
        else:
            return haystack[i:]

 

 

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