P2514【后缀数组】Best Cow Line  

问题描述
FJ is about to take his N (1 ≤ N ≤ 100,000) cows to the annual"Farmer of the Year" competition. In this contest every farmer arranges his cows in a line and herds them past the judges.
The contest organizers adopted a new registration scheme this year: simply register the initial letter of every cow in the order they will appear (i.e., If FJ takes Bessie, Sylvia, and Dora in that order he just registers BSD). After the registration phase ends, every group is judged in increasing lexicographic order according to the string of the initials of the cows' names.
FJ is very busy this year and has to hurry back to his farm, so he wants to be judged as early as possible. He decides to rearrange his cows, who have already lined up, before registering them.
FJ marks a location for a new line of the competing cows. He then proceeds to marshal the cows from the old line to the new one by repeatedly sending either the first or last cow in the (remainder of the) original line to the end of the new line. When he's finished, FJ takes his cows for registration in this new order.
Given the initial order of his cows, determine the least lexicographic string of initials he can make this way.
题目大意：给定一个字符数组，可以从数组的开头或者结尾取出元素，按取出顺序排成一列，使得他的字典序最小。
输入格式
第一行一个整数N(N<=100000)
接下来N行每行一个大写字母，第i+1行是第i头牛的名字首字母
输出格式
共一行，N个大写字母，表示N头牛的新的排列顺序
样例输入
6
A
C
D
B
C
B
样例输出
ABCBCD