80. Remove Duplicates from Sorted Array II — Python beats 98.40% using collections.Counter!

Python's collections.Counter() counts elements in a list and returns a dictionary.

For example, nums = [1, 1, 1, 2, 2, 3] → Counter({1: 3, 2: 2, 3: 1}).

1. Traverse the dictionary: when value > 2, set value = 2. This caps any element appearing more than twice down to 2.
2. Convert Counter({1: 2, 2: 2, 3: 1}) back to a list using elements().

Since the problem passes nums by reference, we just need to clear nums and extend it with the modified list.

from collections import Counter # import
class Solution:
    def removeDuplicates(self, nums: List[int]) -> List[int]:
        dict1 = Counter(nums)
        for i in dict1:
            if dict1[i] > 2:
                dict1[i] = 2
        list1 = list(dict1.elements())
        nums.clear() # clear the list
        nums.extend(list1) # add elements back
        return len(nums)

Complexity Analysis

Time complexity: O(n), where n is the length of the array. We traverse the array at most once.

Space complexity: O(1). We only use constant space for a few variables.

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