CF724D Dense Subsequence

给定长度为n(1

输入包括两行

输出仅一行，为重排列后的选中字符串

In the first sample, one can choose the subsequence $ {3} $ and form a string "a". In the second sample, one can choose the subsequence $ {1,2,4} $ (symbols on this positions are 'a', 'b' and 'a') and rearrange the chosen symbols to form a string "aab".