在列表中查找仅在列表中出现一次的元素

def find_single_element(lst):
    element_count = {}
    for element in lst:
        if element in element_count:
            element_count[element] += 1
        else:
            element_count[element] = 1
    
    for element, count in element_count.items():
        if count == 1:
            return element

# 示例
lst = [2, 2, 1, 3, 3, 4, 4]
print(find_single_element(lst))  # 输出: 1

def find_single_element_xor(lst):
    result = 0
    for element in lst:
        result ^= element
    return result

# 示例
lst = [2, 2, 1, 3, 3, 4, 4]
print(find_single_element_xor(lst))  # 输出: 1

def find_single_element_sort(lst):
    lst.sort()
    for i in range(0, len(lst)-1, 2):
        if lst[i] != lst[i+1]:
            return lst[i]
    return lst[-1]

# 示例
lst = [2, 2, 1, 3, 3, 4, 4]
print(find_single_element_sort(lst))  # 输出: 1