LeetCode-Intersection of Two Arrays II

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

Example 1:

Input: nums1 = [1,2,2,1], nums2 = [2,2]
Output: [2,2]

Example 2:

Input: nums1 = [4,9,5], nums2 = [9,4,9,8,4]
Output: [4,9]

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?

题意：求两个数组的交集，可能包括重复的元素；

解法：考虑利用哈希表来实现，我们首先将其中一个数组的所有元素按照<元素，出现次数>来存储，之后遍历另外一个数组，每次执行以下的操作

• 如果哈希表中此元素的次数大于0，则当前元素为交集的元素；
• 如果哈希表中此元素的次数小于或等于0，则交集中不存在此元素或两个数组中此元素的个数不相同；

Java

class Solution {
    public int[] intersect(int[] nums1, int[] nums2) {
        Map frequency = new HashMap<>();
        List res = new ArrayList<>();
        int cnt = 0;
        for (int num: nums1) {
            frequency.put(num, frequency.getOrDefault(num, 0) + 1);
        }
        for (int num: nums2) {
            frequency.put(num, frequency.getOrDefault(num, 0) - 1);
            if (frequency.get(num) >= 0) {
                res.add(num);
            }
        }
        return res.stream().mapToInt(Integer::valueOf).toArray();
    }
}