350. Intersection of Two Arrays II 

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class Solution {
    public int[] intersect(int[] nums1, int[] nums2) {
        Arrays.sort(nums1);
        Arrays.sort(nums2);

        List<Integer> list = new ArrayList<>();
        int index1 = 0, index2 = 0;
        while (index1 < nums1.length && index2 < nums2.length) {
            int num1 = nums1[index1];
            int num2 = nums2[index2];

            if (num1 < num2) {
                index1++;
            } else if (num1 > num2) {
                index2++;
            } else {
                list.add(num1);

                index1++;
                index2++;
            }
        }

        int[] array = new int[list.size()];
        for (int i = 0; i < list.size(); i++) {
            array[i] = list.get(i);
        }
        return array;
    }
}

References

350. Intersection of Two Arrays II