如何在字符串中查找唯一字母

def find_unique_char(s):
    char_count = {}
    for char in s:
        if char in char_count:
            char_count[char] += 1
        else:
            char_count[char] = 1
    
    for char in s:
        if char_count[char] == 1:
            return char
    return None

# 示例
print(find_unique_char("aabbccddeeffg"))  # 输出: g

def find_unique_char(s):
    char_count = [0] * 128  # ASCII码表共有128个字符
    for char in s:
        char_count[ord(char)] += 1
    
    for char in s:
        if char_count[ord(char)] == 1:
            return char
    return None

# 示例
print(find_unique_char("aabbccddeeffg"))  # 输出: g

def find_unique_char(s):
    checker = 0
    for char in s:
        val = ord(char) - ord('a')
        if (checker & (1 << val)) > 0:
            checker ^= (1 << val)
        else:
            checker |= (1 << val)
    
    for char in s:
        val = ord(char) - ord('a')
        if (checker & (1 << val)) > 0:
            return char
    return None

# 示例
print(find_unique_char("aabbccddeeffg"))  # 输出: g