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

思路：

1. 找出所有字符串共同的前缀。以第一个字符串为标的，依次进行比较。
2. 设置共同前缀的index为0，最小长度是所有字符串长度中最小的那个。依次比较strs中的元素，如果不等，就返回第一个字符串到index长度的slice。index加一，直至大于最小长度。

#!/usr/bin/env python
# -*- coding: UTF-8 -*-

class Solution(object):
    def longestCommonPrefix(self, strs):
        """
        :type strs: List[str]
        :rtype: str
        """
        if len(strs) == 0:
            return ""

        longest = len(strs[0])
        for e in strs[1:]:
            index = 0
            while index < len(e) and index < longest and strs[0][index] == e[index]:
                index += 1
            longest = min(index, longest)
        return strs[0][:longest]

    def longestCommonPrefix2(self, strs):
        if len(strs) <= 1:
            return strs[0] if len(strs)==1 else ""

        end, minlength = 0, min([len(s) for s in strs])
        while (end < minlength):
            for i in xrange(1, len(strs)):
                if strs[i][end] != strs[i-1][end]:
                    return strs[0][:end]
            end += 1
        return strs[0][:end]

if __name__ == '__main__':
    sol = Solution()

    strs = ["abcd", "abcde", "abcdef"]
    strs = ["a", "b"]

    print sol.longestCommonPrefix(strs)
    print sol.longestCommonPrefix2(strs)