Python:将第一个第n个匹配字母替换为另一个字母

def replace_letters(string, old_letter, new_letter, n):
    # 找到第一个匹配的字母
    first_index = string.find(old_letter)
    if first_index == -1:
        return string
    
    # 找到第n个匹配的字母
    count = 1
    for i in range(first_index + 1, len(string)):
        if string[i] == old_letter:
            count += 1
            if count == n:
                last_index = i
                break
    else:
        return string
    
    # 替换字母
    new_string = string[:first_index] + new_letter + string[first_index + 1:last_index] + new_letter + string[last_index + 1:]
    return new_string

# 示例用法
string = "Hello, World!"
old_letter = "o"
new_letter = "x"
n = 2
new_string = replace_letters(string, old_letter, new_letter, n)
print(new_string)