leetcode 14

解题思路：min和max

在Python里字符串是可以比较的，按照ascII值排，举例abb， aba，abac，最大为abb，最小为aba。所以只需要比较最大最小的公共前缀就是整个数组的公共前缀

min(["flower","flow","flight"])  #flight
max(["flower","flow","flight"])  #'flower'
 
for i,x in enumerate(min(["flower","flow","flight"])):
    print (i,x)
'''
0 f
1 l
2 i
3 g
4 h
5 t
'''

def longestCommonPrefix(self, strs) :
    if not strs: 
        return ""
    s1 = min(strs)
    s2 = max(strs)
    for i,x in enumerate(s1):
        if x != s2[i]:
            return s2[:i]
    return s1