richie_rich.py

# https://www.hackerrank.com/challenges/richie-rich/problem

def highestValuePalindrome(LisT, n, changes):
    number = list(LisT)
    oddString = True if n % 2 != 0 else False

    # Main loop starts in the middle:
    left = n // 2 - 1
    right = left + 1 if not oddString else left + 2

    # Inspecting, for remaining changes, how many are needed to '9' changes:
    nines = [0 for _ in range(left + 1)]
    for i in range(left + 1):
        if number[i] != "9" and number[-1 - i] != "9":
            nines[i] = 2 if i == 0 else 2 + nines[i - 1]
        elif number[i] != "9" or number[-1 - i] != "9":
            nines[i] = 1 if i == 0 else 1 + nines[i - 1]
        else:
            nines[i] = nines[i - 1]

    # Function to change to '9':
    def allToNine(List, lft, changes):
        for i in range(lft + 1):
            if List[i] != "9" and List[-1 - i] != "9":
                List[i] = List[-1 - i] = "9"
                changes -= 2
            elif List[i] != "9" or List[-1 - i] != "9":
                List[i] = List[-1 - i] = "9"
                changes -= 1
        return changes

    # Main loop:
    while changes >= 0 and left >= 0:
        if nines[left] <= changes:
            changes = allToNine(number, left, changes)
            break
        # If elements are different, use a change to set them to the maximum:
        if number[left] != number[right]:
            if changes <= 0:
                return "-1"  # The lenght of string may not be altered!
            else:
                number[left] = number[right] = max(number[left], number[right])
                changes -= 1
        left -= 1
        right += 1

    # For strings with odd length, if any change remains, change the center:
    if changes >= 1 and oddString:
        number[n // 2] = "9"

    return "".join(number)