Given two arrays, write a function to compute their intersection.

Example:

Given nums1 = [1, 2, 2, 1], nums2 = [2, 2], return [2, 2].

Note:

Each element in the result should appear as many times as it shows in both arrays. The result can be in any order.

Follow up:

• What if the given array is already sorted? How would you optimize your algorithm?
• What if nums1’s size is small compared to nums2’s size? Which algorithm is better?
• What if elements of nums2 are stored on disk, and the memory is limited such that you cannot load all elements into the memory at once?

Solution

• 统计nums1 出现次数
• 遍历Num2 插入i 若counts[i] >0， 并更新counts[i]

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class Solution(object):
    def intersect(self, nums1, nums2):
        """
        :type nums1: List[int]
        :type nums2: List[int]
        :rtype: List[int]
        """
        # counts 存了nums1[i]出现的次数
        counts = collections.Counter(nums1)
        ret = []
        for i in nums2:
            if counts[i] > 0:
                ret.append(i)
                counts[i] -= 1
        return ret