Question

Python3: Write the function longestSubpalindrome(s), that takes a string s and returns the longest palindrome that occurs as consecutive characters (not just letters, but any characters) in s.

so longestSubpalindrome("ab-4-be!!!") returns "b-4-b".If there is a tie, return the lexicographically larger value -- in Python, a string s1 is lexicographically greater than a string s2 if (s1 > s2).

So: longestSubpalindrome("abcbce") returns "cbc", since ("cbc" > "bcb"). Note that unlike the previous functions, this function is case-sensitive (so "A" is not treated the same as "a" here).

Also, from the explanation above, we see that longestSubpalindrome("aba") is "aba", and longestSubpalindrome("a") is "a".

Restrictions: for loop, slicing, 'in' keyword, list comprehension, lambda function can't be used. It is recommended to use while loop to solve this problem.

hints: pick all possible starting and ending positions for a substring, and verify if it is a palindrome.

Answer 1

Please find the python to find the longest Sub Palindome from a given string in the function argument.

Note:

1. We can count the string length using string indices and stop counting when there is no more characters left.

2. This program will return the same character if the length of the given string is 1

3. If the given length of string is more than 1 and no palindrome found then it returns None.

4. I have used two apis to calculate the count and reverse of a string

Program:

#method to calculate the string length
def stringCount(string):
    count=0
    #count the number of characters in the string
    while True:
        try:
           count=count+1
           if not string[count]:
              break
        except IndexError:
           break
    return count

#api to revrse the string
def stringReverse(string):
  
    count = stringCount(string)
    temp=""
    count = count -1
    while count >= 0:
        temp=temp+string[count]
        count = count -1
  
    return temp

#method to find the longestSubpalindrome
def longestSubpalindrome(string):
    oriCount = stringCount(string)
    index = 0
    innerIndex = 0
  
    curPalindrome = ""  

    if oriCount == 1:
        return string
  
    prevPalindrome = string[index]
    curPalindrome = string[index]
    index=0
  
    #outer loop to iterate over the string
    while index < oriCount:
        #print(string[index])
        innerIndex = index
        if index != 0:
            curPalindrome = ""
      
        #inner loop to indentify the sub palindrome  
        while innerIndex < oriCount:
            if innerIndex != 0:
                curPalindrome = curPalindrome+ string[innerIndex]
          
            #if its a palindrome then check previous palindrome matches with current
            if curPalindrome == stringReverse(curPalindrome):
                #following check indentifies the which palindrome is greater in size
                if stringCount(prevPalindrome) < stringCount(curPalindrome):
                    prevPalindrome = curPalindrome
                elif stringCount(prevPalindrome) == stringCount(curPalindrome):
                    if prevPalindrome < curPalindrome:
                        prevPalindrome = curPalindrome
                      
            innerIndex = innerIndex + 1
      
        index = index + 1
  
    if stringCount(prevPalindrome) == 1:
        #return none because the string doesnt have prevPalindrome
        return None
      
    return prevPalindrome

#testcases to validate the string whether it has longest sub palindrome

#following are palidrome
print(longestSubpalindrome("a"))
print(longestSubpalindrome("abacbbc"))
print(longestSubpalindrome("ab-4-be!!!"))
print(longestSubpalindrome("abcbce"))
print(longestSubpalindrome("NIXIN"))

#following are not a palidrome and returns None
print(longestSubpalindrome("Lebron"))
print(longestSubpalindrome("Kyrie"))

Output:

a
cbbc
b-4-b
cbc
NIXIN
None
None

Screen Shot: