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# Intersection of Two Arrays II

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

## Example

**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?
* 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?

## Note

这里不同的结果需要是重复的

## Code

使用排序加双指针

```java
public int[] intersect2(int[] nums1, int[] nums2) {
    List<Integer> res = new ArrayList<Integer>();
    Arrays.sort(nums1);
    Arrays.sort(nums2);
    for(int i = 0, j = 0; i< nums1.length && j<nums2.length;){
            if(nums1[i]<nums2[j]){
                i++;
            }
            else if(nums1[i] == nums2[j]){
                res.add(nums1[i]);
                i++;
                j++;
            }
            else{
                j++;
            }
    }
    int[] result = new int[res.size()];
    for(int i = 0; i<res.size();i++){
        result[i] = res.get(i);
    }
    return result;
}
```

用counting map

```java
public int[] intersect(int[] nums1, int[] nums2) {
    List<Integer> res = new ArrayList<Integer>();
    HashMap<Integer, Integer> map = new HashMap<>();

    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])){
            if(map.get(nums2[i]) > 0){
                res.add(nums2[i]);
                map.put(nums2[i], map.get(nums2[i]) - 1);
            }
        }
    }

    int[] result = new int[res.size()];
    for(int i = 0; i<res.size();i++){
        result[i] = res.get(i);
    }
    return result;
}
```