Strings — Longest Common Prefix

solve(strs) — return the longest string that is a prefix of every string in
strs (non-empty array), or "" if there is none.

Take the first string as the candidate prefix and shrink it until each other string starts
with it. Or compare column by column: the prefix ends at the first index where any string is
shorter or disagrees with the first one. Either way is O(total characters).

Examples

solve(["flower","flow","flight"])   → "fl"
solve(["dog","racecar","car"])      → ""
solve(["a"])                        → "a"