Richness of binary words
题目链接：
http://acm.hust.edu.cn/vjudge/contest/126823#problem/B
Description
For each integer i from 1 to n, you must print a string s i of length n consisting of letters ‘a’ and ‘b’ only. The string s i must contain exactly i distinct palindrome substrings. Two substrings are considered distinct if they are different as strings.
The input contains one integer n (1 ≤ n ≤ 2000).
Output
You must print n lines. If for some i, the answer exists, print it in the form “ i : s i” where s i is one of possible strings. Otherwise, print “ i : NO”.
input output
4
1 : NO
2 : NO
3 : NO
4 : aaaa
##题意：
要求输出n个长度为n的字符串(只能放a或b)：
要求Si中不同回文子串的个数恰为i.
##题解：
本质就是找出一种构造方式使得回文子串个数不会再增加.
参考2015-ICPC合肥现场赛H题：
http://blog.csdn.net/snowy_smile/article/details/49870109
http://blog.csdn.net/keshuai19940722/article/details/49839359
##代码：
``` cpp
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