Write a function to find the longest common prefix string amongst an array of strings.
Longest common prefix for a pair of strings S1 and S2 is the longest string S which is the prefix of both S1 and S2.
As an example, longest common prefix of
Given the array of strings, you need to find the longest S which is the prefix of ALL the strings in the array.
Given the array as:
[ "abcdefgh", "aefghijk", "abcefgh" ]
The answer would be “a”.