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Hello, my homework asked to make a program that accepts a string as a parameter and that returns the letters that appeared the most without any duplicates. I would just like to know if there is any way I could of used any list comprehensions or optimized it in any way? Thanks!

# Gets the max number in the list
def maxNumber(L):
    i = 0 
    maxNum = L[0]
    while i < len(L):
        if L[i] >= maxNum:
            maxNum = L[i]
        i += 1
    return maxNum

# Returns a index list on the letters who appeared the most
def indexes(maxNum,S):
    index = []
    j = 0 
    for i in S:
        if S.count(i) == maxNum:
            index.append(j)
        j += 1
    return index 

# Returns the letters that appeared the most
def maxLetters(index,S):
    maxLetters = ''
    for i in index: 
        if S[i] not in maxLetters:
            maxLetters += S[i]
    return maxLetters

# Main Function 
def CharacterCount(S):
    # Removes Spaces
    S = S.replace(' ','')
    # Max Number - List Comprehension returns how many time times a letter appears
    maxNumbers = maxNumber([S.count(i) for i in S])
    # Returns a index list on the letters who appeared the most
    index = indexes(maxNumbers,S)
    # Returns the letters that appeared the most 
    maxLetter = maxLetters(index,S)
    print(maxLetter)

CharacterCount('Python is great fun')
CharacterCount('Good Times')

to get character count:

from collections import Counter

def CharacterCount(S):
    S = S.strip().replace(' ','')
    return Counter(S)

print(f"\nCounts 'Python is great fun': {CharacterCount('Python is great fun')}")
print(f"Counts 'Good Times': {CharacterCount('Good Times')}\n")

Output:
Counts 'Python is great fun': Counter({'t': 2, 'n': 2, 'P': 1, 'y': 1, 'h': 1, 'o': 1, 'i': 1, 's': 1, 'g': 1, 'r': 1, 'e': 1, 'a': 1, 'f': 1, 'u': 1})
Counts 'Good Times': Counter({'o': 2, 'G': 1, 'd': 1, 'T': 1, 'i': 1, 'm': 1, 'e': 1, 's': 1})