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

Example:
Given nums1 = [1, 2, 2, 1], nums2 = [2, 2], return [2, 2].

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?

这道题用hashmap存下每个数字的数量即可

public class Solution {
    public int[] intersect(int[] nums1, int[] nums2) {
        HashMap<Integer,Integer> map = new HashMap();
        ArrayList list = new ArrayList();
        for(int i=0;i<nums1.length;i++){
            if(map.containsKey(nums1[i])){
                map.put(nums1[i],map.get(nums1[i])+1);
            }
            else{
                map.put(nums1[i],1);
            }
        }
        for(int i=0;i<nums2.length;i++){
            if(map.containsKey(nums2[i])){
                map.put(nums2[i],map.get(nums2[i])-1);
                if(map.get(nums2[i]) == 0){
                    map.remove(nums2[i]);
                }
                list.add(nums2[i]);
            }
        }
        int[] arr = convertIntegers(list);
        return arr;
    }
    public int[] convertIntegers(List<Integer> integers){
        int[] ret = new int[integers.size()];
        for (int i=0; i < ret.length; i++)
        {
            ret[i] = integers.get(i).intValue();
        }
        return ret;
    }
}