2000. Reverse Prefix of Word

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class Solution:
    def reversePrefix(self, word: str, ch: str) -> str:
        
        def reverseWord(i, j):
            while i < j:
                word[i], word[j] = word[j], word[i]
                i += 1
                j -= 1

        word = [c for c in word]
        n = len(word)
        slow = 0
        fast = 0
        seen = False
        while fast < n:
            if word[fast] == ch and not seen:
                reverseWord(slow, fast)
                seen = True
            fast += 1
        
        return ''.join(word)