leetcode 350. Intersection of Two Arrays II

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?

class Solution(object):
    def intersect(self, nums1, nums2):
        """
        :type nums1: List[int]
        :type nums2: List[int]
        :rtype: List[int]
        """
        # use sort, or set(Counter)
        ans = []
        cnt = collections.Counter(nums1)
        for n in nums2:
            if n in cnt and cnt[n] > 0:
                cnt[n] -= 1
                ans.append(n)
        return ans

可以更精简些：

from collections import Counter

class Solution(object):
    def intersect(self, nums1, nums2):
        c1, c2 = Counter(nums1), Counter(nums2)
        return sum([[num] * min(c1[num], c2[num]) for num in c1 & c2], [])

自己写的话：

class Solution(object):
    def intersect(self, nums1, nums2):

        counts = {}
        res = []

        for num in nums1:
            counts[num] = counts.get(num, 0) + 1

        for num in nums2:
            if num in counts and counts[num] > 0:
                res.append(num)
                counts[num] -= 1

        return res

注意：

class Solution(object):
    def intersect(self, nums1, nums2):
        """
        :type nums1: List[int]
        :type nums2: List[int]
        :rtype: List[int]
        """
        # use sort, or set(Counter)
        return list(set(nums1) & set(nums2))

是不可以的，因为set里是没有重复数据的：

Input: [1,2,2,1] [2,2]

Output: [2]

Expected: [2,2]

另外就是使用sort，归并思路：

class Solution(object):
    def intersect(self, nums1, nums2):
        """
        :type nums1: List[int]
        :type nums2: List[int]
        :rtype: List[int]
        """
        # use sort, or set(Counter)
        nums1.sort()
        nums2.sort()
        i = j = 0
        ans = []
        l1, l2 = len(nums1), len(nums2)
        while i < l1 and j < l2:
            if nums1[i] == nums2[j]:
                ans.append(nums1[i])
                i += 1
                j += 1
            elif nums1[i] < nums2[j]:
                i += 1
            else:
                j += 1
        return ans