如何检查字符串中要查找的单词的前一个字母是否存在？递归

def check_previous_letter(word, string):
    if len(string) < len(word):
        return False
    elif string[:len(word)] == word:
        if len(string) == len(word):
            return False
        elif string[len(word)-1].isalpha():
            return True
    return check_previous_letter(word, string[1:])

# 示例用法
word = "apple"
string = "I have an apple"
result = check_previous_letter(word, string)
print(result)  # 输出：True