如果你wa on 13

Lexicographical order is defined in following way. When we compare s and t , first we find the leftmost position with differing characters: $s_{i}$ !=$t_{i}$ If there is no such position (i. e. s is a prefix of t or vice versa) the shortest string is less.

若没有那样一个位置，更短的字符串字典序更小。