Check if a given string is a rotation of a palindrome | GeeksforGeeks

Related LeetCode - Longest Palindromic Substring
Given a string, check if it is a rotation of a palindrome. For example your function should return true for “aab” as it is a rotation of “aba”.

An Optimized Solution can work in O(n) time. The idea is similar to this post. Following are steps.
1) Let the given string be ‘str’ and length of string be ‘n’. Create a temporary string ‘temp’ which is of size 2n and contains str followed by str again. For example, let str be “aab”, temp would be “aabaab”.
2) Now the problem reduces to find a palindromic substring of length n in str. If there is palindromic substring of length n, then return true, else return false.
We can find whether there is a palindromic substring of size n or not in O(n) time (See Longest palindromic substring)

Manacher’s Algorithm: It takes advantages of palindrome's symmetric property and avoids some unnecessary computations.
http://massivealgorithms.blogspot.com/2014/06/leetcode-longest-palindromic-substring.html

http://www.techiedelight.com/check-given-string-rotated-palindrome-not/

The problem is similar to finding Longest Palindromic Substring problem. Let the given string be S of length m. The idea is to concatenate the given string with itself i.e (S = S + S) and find palindromic substring of length m in the modified string (S + S). If palindromic substring of length m exists in the modified string, we return true else we return false.

// expand in both directions of low and high to find

// palindrome of length k

bool expand(string str, int low, int high, int k)

{

    // run till str[low.high] is a palindrome

    while (low >= 0 && high < str.length() && (str[low] == str[high]))

    {

        // return true palindrome of length k is found

        if (high - low + 1 == k)

            return true;

        // expand in both directions

        low--, high++;

    }

    // return false if palindrome of length k is not found

    return false;

}

// Function to check if palindromic substring of length k exists or not

bool LongestPalindromicSubstring(string str, int k)

{

    for (int i = 0; i < str.length() - 1; i++)

        // check if odd length or even length palindrome of length k

        // exists which have str[i] as its mid point

        if (expand(str, i, i, k) || expand(str, i, i + 1, k))

            return true;

    

    return false;

}

// Function to check if given string is a rotated palindrome or not

bool isRotatedPalindrome(string str)

{

    // length of given string

    int m = str.length();

    

    // return true if longest palindromic substring of length m

    // exists in the string (str + str)

    return LongestPalindromicSubstring(str + str, m);

}

A Simple Solution is to take the input string, try every possible rotation of it and return true if a rotation is a palindrome. If no rotation is palindrome, then return false.

bool isRotationOfPalindrome(string str)

{

   // If string iteself is palindrome

   if (isPalindrome(str))

         return true;

   // Now try all rotations one by one

   int n = str.length();

   for (int i = 0; i < n-1; i++)

   {

       string str1 = str.substr(i+1, n-i-1);

       string str2 = str.substr(0, i+1);

       // Check if this rotation is palindrome

       if (isPalindrome(str1.append(str2)))

          return true;

   }

   return false;

}

http://www.geeksforgeeks.org/recursive-function-check-string-palindrome/

bool isPalRec(char str[], int s, int e)

{

    // If there is only one character

    if (s == e)

       return true;

 

    // If first and last characters do not match

    if (str[s] != str[e])

       return false;

 

    // If there are more than two characters,

    // check if middle substring is also

    // palindrome or not.

    if (s < e+1)

       return isPalRec(str, s+1, e-1);

 

    return true;

}

void isPalindrome(char str[])

{

    // Start from leftmost and rightmost corners of str

    int l = 0;

    int h = strlen(str) - 1;

 

    // Keep comparing characters while they are same

    while (h > l)

    {

        if (str[l++] != str[h--])

        {

            printf("%s is Not Palindrome\n", str);

            return;

        }

    }

    printf("%s is palindrome\n", str);

}

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