Find index i such that prefix of S1 and suffix of S2 till i form a palindrome when concatenated in Python

Suppose we have two strings S1 and S2 of same lengths, we have to find an index i such that S1[0…i] and S2[i+1…n-1] give a palindrome when they are concatenated together. When it is not possible, return -1.

So, if the input is like S1 = "pqrsu", S2 = "wxyqp", then the output will be 1 as S1[0..1] = "pq", S2[2..n-1] = "ypq", then S1 + S2 = "pqyqp" indicates is a palindrome.

To solve this, we will follow these steps −

• n := size of str1

• str := blank string

• for i in range 0 to n, do

  • str := str concatenate str1[i]

  • temp := blank string

  • for j in range i + 1 to n, do

    • temp := temp concatenate str2[j]

  • if isPalindrome(str concatenate temp) is true, then

    • return i

• return -1

Example

Let us see the following implementation to get better understanding −

 Live Demo

def isPalindrome(s):
   if s == s[::-1]:
      return True
   return False
def find_index(str1, str2):
   n = len(str1)
   str = ""
   for i in range(n):
      str = str + str1[i]
      temp = ""
      for j in range(i + 1, n):
         temp += str2[j]
      if (isPalindrome(str + temp)):
         return i
   return -1
str1 = "pqrsu"
str2 = "wxyqp"
print(find_index(str1, str2))

Input

"pqrsu", "wxyqp"

Output

1

Arnab Chakraborty

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Updated on: 25-Aug-2020

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