确定一个字符串是否是另一个字符串的子集。不要使用contains或in关键字

def is_subset(subset, main_string):
    subset_index = 0
    for char in main_string:
        if subset_index < len(subset) and char == subset[subset_index]:
            subset_index += 1
    return subset_index == len(subset)

# 示例使用
main_string = "hello world"
subset = "low"
print(is_subset(subset, main_string))  # 输出应该是 True

def is_subset_case_insensitive(subset, main_string):
    subset = subset.lower()
    main_string = main_string.lower()
    subset_index = 0
    for char in main_string:
        if subset_index < len(subset) and char == subset[subset_index]:
            subset_index += 1
    return subset_index == len(subset)

# 示例使用
main_string = "Hello World"
subset = "low"
print(is_subset_case_insensitive(subset, main_string))  # 输出应该是 True