Given a string, count how many maximum-length palindromes are present.(It need not be a substring)
Input : str = "ababa" Output: 2 Explanation : palindromes of maximum of lenghts are : "ababa", "baaab" Input : str = "ababab" Output: 4 Explanation : palindromes of maximum of lenghts are : "ababa", "baaab", "abbba", "babab"
Approach A palindrome can be reprsented as “str + t + reverse(str)”.
Note: “t” is empty for even length palindromic strings.
Calculate in how many ways “str” can be made and then multiply with “t” (number of single characters left out).
Let ci be the number of occurrences of character in the string. Consider the following cases:
1. If ci is even. Then a half of every maximum palindrome will contain exactly letters fi = ci / 2.
2.If ci is odd. Then a half of every maximum palindrome will contain exactly letters fi = (ci – 1)/ 2.
Let k be the number of odd ci. If k=0, the maximum palindromes length will be even; otherwise it will be odd and there will be exactly k possible middle letters i.e., we can set this letter to the middle of palindrome.
The number of permutations of n objects with n1 identical objects of type 1, n2 identical objects of type 2,, and n3 identical objects of type 3 is n! / (n1! * n2! * n3!) .
So here we have total number of characters as fa+fb+fa+…….+fy+fz . So number of permutation is (fa+fb+fa+…….+fy+fz+)! / fa! fb!…fy!fz!.
Now If K is not 0, it’s obvious that the answer is k * (fa+fb+fa+…….+fy+fz+)! / fa! fb!…fy!fz!
Below is the implementation of the above.
Time Complexity : O(n)
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