Easy to understand and O(n+m) 5ms fast using single HashSet

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    The basic idea is to create a quick lookup to check whether an item is duplicated in both arrays.
    set is this lookup, which is built from the shorter array anticipating less distinct elements in that. This may be premature and obviously easy to trick: [1000 times 1], [1...100], but I think if we take a randomly distributed input it'll be faster most of the time.

    I've seen other solutions doing if (contains) remove which does two hash lookups, while using the return value of remove (see JavaDoc) can "reuse" the result of contains and remove it immediately.

    I used a primitive array instead of an ArrayList to prevent boxing and the amortized growth. Clamping the array with arraycopy should be better than converting a List to a primitive array.
    Notice that the temporary intersection array is allocated with the size of the smaller array. This is because it's not possible to have more distinct items than the length of the container.

    public class Solution {
        public int[] intersection(int[] nums1, int[] nums2) {
            int[] larger = nums1.length < nums2.length? nums2 : nums1;
            int[] smaller = nums1.length < nums2.length? nums1 : nums2;
            Set<Integer> set = new HashSet<Integer>();
            for (int num : smaller) {
            int[] intersection = new int[smaller.length];
            int i = 0;
            for (int num : larger) {
                if (set.remove(num)) {
                    intersection[i++] = num;
            int[] result = new int[i];
            System.arraycopy(intersection, 0, result, 0, i);
            return result;

    I'm curious to hear thoughts about the optimizations. Do you think they're premature? (assume evenly distributed random inputs)

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