Leetcode350-intersectionOfTwoArraysII

Description

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

Example

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?

Solution

• Time Complexity: $O(N)$
• Space Complexity: $O(N)$

class Solution {
    public int[] intersect(int[] nums1, int[] nums2) {
        HashMap<Integer, Integer> map = new HashMap<>();
        List<Integer> res = new ArrayList<>();
        for (int num: nums1){
            map.put(num, map.getOrDefault(num, 0) + 1);
        }
        for (int num: nums2){
            if (map.containsKey(num) && map.get(num) > 0){
                map.put(num, map.get(num) - 1);
                res.add(num);
            }
        }
        int[] ans = new int[res.size()];
        for (int i = 0; i < res.size(); i++)
            ans[i] = res.get(i);
        return ans;
    }
}

Follow up

1. Two pointer iteration.
2. Let’s say nums1 is K size. Then we should do binary search for every element in nums1. Each lookup is O(log N), and if we do K times, we have O(K log N). If K this is small enough, O(K log N) < O(max(N, M)). Otherwise, we have to use the previous two pointers method.
3. External Merge-Sort / MapReduced?

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