[LeetCode][Python]Intersection of Two Arrays II

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 num2'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?

https://leetcode.com/problems/intersection-of-two-arrays-ii/

---

把一个数组中的元素的元素对应到哈希表，key是值，value是出现的次数，然后对照哈希表遍历另一个数组，O(n)。

follow up的问题：

1. 两个数组都有序的话可以用双指针；

2. 把num1做成哈希表，数量比较少；

3. 也是把num1做成哈希表，nums2比较多就读一点处理一点。

class Solution(object):
    def intersect(self, nums1, nums2):
        """
        :type nums1: List[int]
        :type nums2: List[int]
        :rtype: List[int]
        """
        res = []; dictionary = {}
        for num in nums1:
            if not dictionary.has_key(num):
                dictionary[num] = 1
            else:
                dictionary[num] += 1
        for num in nums2:
            if dictionary.has_key(num) and dictionary[num] > 0:
                dictionary[num] -= 1
                res.append(num)
        return res;