Write a function to find the longest common prefix string amongst an array of strings.

longestcommonprefix.py

class Solution:
    # @return a string
    def longestCommonPrefix(self, strs):
        # computes the LCP of the stringlist strs
        if strs is None or len(strs) == 0:
            return ""
        
        return reduce(self.lcp, strs)
            
    def lcp(self, str1, str2):
        # computes the LCP of str1 and str2
        for index, (char1, char2) in enumerate(zip(str1, str2)):
            if char1 != char2:
                return str1[:index]
                
        return str1[:min(len(str1), len(str2))]