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.

• 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?

# Solution

class Solution {
public int[] intersect(int[] nums1, int[] nums2) {
Map<Integer, Integer> count1 = new HashMap<>();

for(int n : nums1) {
if(count1.containsKey(n)) {
count1.put(n, count1.get(n) + 1);
} else {
count1.put(n, 1);
}
}
ArrayList<Integer> intersection = new ArrayList<>();
for(int n : nums2) {
if(count1.containsKey(n)) {
if(count1.get(n) > 1) {
count1.put(n, count1.get(n) - 1);
} else {
count1.remove(n);
}
}
}
int[] res = new int[intersection.size()];
int i = 0;
for(int n: intersection) {
res[i++] = n;
}
return res;
}
}