2365: Ann and Half-Palindrome
Time Limit: 2 Sec Memory Limit: 256 MBSubmit: 0 Solved: 0
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Description
Tomorrow Ann takes the hardest exam of programming where she should get an excellent mark.
On the last theoretical class the teacher introduced the notion of a half-palindrome.
String t is a half-palindrome, if for all the odd positions i (
) the following condition is held: ti=t|t|-i+1, where |t| is the length of string t if positions are indexed from 1. For example, strings "abaa", "a", "bb", "abbbaa" are half-palindromes and strings "ab", "bba" and "aaabaa" are not.
Ann knows that on the exam she will get string s, consisting only of letters a and b, and number k. To get an excellent mark she has to find the k-th in the lexicographical order string among all substrings of s that are half-palyndromes. Note that each substring in this order is considered as many times as many times it occurs in s.
The teachers guarantees that the given number k doesn't exceed the number of substrings of the given string that are half-palindromes.
Can you cope with this problem?
The first line of the input contains string s (1≤|s|≤5000), consisting only of characters 'a' and 'b', where |s| is the length of string s.
The second line contains a positive integer k− the lexicographical number of the requested string among all the half-palindrome substrings of the given string s. The strings are numbered starting from one.
It is guaranteed that number k doesn't exceed the number of substrings of the given string that are half-palindromes.
Print a substring of the given string that is the k-th in the lexicographical order of all substrings of the given string that are half-palindromes.
abbabaab
7
abaa
aaaaa
10
aaa
bbaabb
13
bbaabb
By definition, string a=a1a2... an is lexicographically less than string b=b1b2... bm, if either a is a prefix of b and doesn't coincide with b, or there exists such i, that a1=b1,a2=b2,... ai-1=bi-1,ai<bi.
In the first sample half-palindrome substrings are the following strings−a, a, a, a, aa, aba, abaa, abba, abbabaa, b, b, b, b, baab, bab, bb, bbab, bbabaab (the list is given in the lexicographical order).