作用：把迭代器中相邻的元素按照key函数分组，当key=None时，把相邻的重复元素进行分组。

5 tips to get the most out of list comprehensions in Python

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Given a string containing only digits, restore it by returning all possible valid IP address combinations.

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique s...

Given two integers n and k, return all possible combinations of k numbers out of 1 … n.

Given a set of candidate numbers (C) (without duplicates) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

Given a collection of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.