14. Longest Common Prefix

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

找出字符串数组的最长公共前缀

Trick：利用排序，只比较第一个和最后一个字符串即可

class Solution(object):    def longestCommonPrefix(self, strs):        if strs == []:            return ""              strs.sort()        str1 = strs[0]        str2 = strs[-1]        common_len = 0        if len(str1) <= len(str2):          cnt = len(str1)        else:          cnt = len(str2)        for i in xrange(cnt):          if str1[i] == str2[i]:            common_len = i+1          else:            break        return str1[:common_len]