http://baike.baidu.com/link?url=kw5Kxe3nSvRJR0TpJUpMrORcQL8fyZFpJlT9_o0RlGYOy0bKFobabPPSj3LxGfy7o1qGVycrYK4Iags3hMFq0a
在组合数合里，贝尔数给出了集合划分的数目，以数学家埃里克·坦普尔·贝尔（Eric Temple Bell）命名，是组合数学中的一组整数数列。[1]
以B0= B1=1为始， 首几项的贝尔数为：1, 1, 2, 5, 15, 52, 203, 877, 4140, 21147, 115975, …（OEIS的A000110数列）
先预处理出来比小于等于n的素数的个数和回文数的个数…
因为字母的排列顺序是任意的,所以判断能否形成回文串的条件就成了出现次数为奇数的字母的个数是否大于1个,如果是,那么一定不能形成回文串,否则一定可以.
C. Replacement
time limit per test
2 seconds
memory limit per test
256 megabytes
input
standard input
output
standard output
Daniel has a string s, consisting of lowercase English letters and period signs (characters ‘.’). Let’s define the operation of replacement as the following sequence of steps: find a substring “..” (two consecutive periods) in string s, of all occurrences of the substring let’s choose the first one, and replace this substring with string “.”. In other words, during the replacement operation, the first two consecutive periods are replaced by one. If string s contains no two consecutive periods, then nothing happens.
