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Question_21.py
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Question_21.py
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'''
Question 21
___________
You are given a palindrome string P of length N consisting of only lowercase letters of the English alphabet. Find the shortest non-empty palindrome string Q such that P concatenated with Q forms a palindrome. Formally, the string PQ forms a palindrome.
Input
The first line of the input gives the number of test cases, T. T test cases follow. Each test case consists of two lines. The first line of each test case contains an integer N denoting the length of the string P. The second line of each test case contains a palindrome string P of length N.
Output
For each test case, output one line containing Case # x: y, where x is the test case number (starting from 1) and y is the non-empty palindrome string Q as described above.
Input
3
4
abba
4
cccc
6
cdccdc
Output:
Case # 1: abba
Case # 2: c
Case # 3: cdc
'''
def manacher(s):
s = '^#' + '#'.join(s) + '#$'
P = [0]*len(s)
C, R = 0, 0
for i in range(1, len(s)-1):
i_mirror = 2*C-i
if R > i:
P[i] = min(R-i, P[i_mirror])
while s[i+1+P[i]] == s[i-1-P[i]]:
P[i] += 1
if i+P[i] > R:
C, R = i, i+P[i]
return P
def matching_palindrome():
N = int(input())
P = input()
p = manacher(P)
l = min(p[i] for i in range(2, (len(p)-2)+1) if p[i] != 0 and p[1+p[i]] == p[i] and p[(len(p)-2)-(N-p[i])] == N-p[i])
return P[:l]
def main():
for case in range(int(input())):
print('Case #%d: %s' % (case+1, matching_palindrome()))
if __name__=='__main__':
main()