[LeetCode] Problem 350 - Intersection of Two Arrays II

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

Example

No.1

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

Output: [2,2]

No.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?

Code

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public int[] intersect(int[] nums1, int[] nums2) {
    Map<Integer, Integer> map = new HashMap<>();
    List<Integer> list = 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);
            list.add(num);
        }
    }

    int[] result = new int[list.size()];

    for (int i = 0; i < result.length; i++)
        result[i] = list.get(i);

    return result;
}