350. Intersection of Two Arrays II

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? Still O(n+m) with Hashmap

• What if nums1's size is small compared to nums2's size? Which algorithm is better? same, unless we use binary sort instead of hashmap , O(nlogm) or O(mlgn)

• 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? Will need to sort the data with chucking the data into proper size so that they can feet the RAM size. Use any of sort arlgo to sort the sub datas. and then use merge sort to sort all the sub-datas. Chuck the proper data size of subData and then do merge sort. and write back the final sorted part into disk util all process is finished. Then do binary search for each element in the smallest array among nums1 and nums2 in order to find the intersection.

Solution

class Solution {
    public int[] intersect(int[] nums1, int[] nums2) {
        Map<Integer, Integer> map = new HashMap<>();
        List<Integer> list = new ArrayList<>();
        
        for(int num : nums1){
            if(map.containsKey(num)){
                int value = map.get(num);
                map.put(num, value+1);
            }else{
                map.put(num, 1);
            }
        }
        
        for(int num : nums2){
            if(map.containsKey(num) && map.get(num) > 0){
                list.add(num);
                int value = map.get(num);
                map.put(num, value-1);
            }
        }
        
        return list.stream().mapToInt(i->i).toArray();
    }
}