Write a function to find the longest common prefix string amongst an array of strings.
If there is no common prefix, return an empty string "".
Example 1:
Input: ["flower","flow","flight"] Output: "fl" Example 2:
Input: ["dog","racecar","car"] Output: "" Explanation: There is no common prefix among the input strings. Note:
All given inputs are in lowercase letters a-z.
选取最短的串去进行比较