You get a list of words strs. Return the longest string that every word starts with. If the words share no first letter, return the empty string "".
All three agree on i, n, t, e, r, then split three ways: v, n, v.
moon begins with m and the others with s, so nothing is shared.
A single word is its own longest prefix.
Constraints
1 <= strs.length <= 2000 <= strs[i].length <= 200strs[i] is made of lowercase English letters only (or is empty).The answer can be no longer than the shortest word, and it ends at the first position where any two words disagree.