class Solution:
    def longestCommonPrefix(self, strs):
        """
        :type strs: List[str]
        :rtype: str
        """
        if not strs: return ""
        strs = sorted(strs, key = len)
        i = 1
        pre = strs[0]
        while i < len(strs):
            if not pre in strs[i] or not strs[i].index(pre) == 0:
                pre = pre[:-1]
                i = 1
            else: i += 1
        return pre