The root element can contain 0-n elements.
根元素可以包含0 - n个元素。
Groups are numbered from 1 to n, whereas group (0) is the complete regular expression.
从1到n对组进行编号，而group(0)是一个完整的正则表达式。
If n = 1, x/n can take on only two values: 0 or 1.
当n=1时,x/n的值只可能等于0和1
Then each person can be friends with anywhere from 0 to n-1 other people.
那么每一个人可以有0 ~n - 1个朋友。
Then each person can be friends with anywhere from 0 to n-1 other people.
那么每一个人可以有0 ~n - 1个朋友。
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