## Intersection of Two Arrays II

07/18/2016 Binary Search Hash Table Two Pointers Sort

## Question

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

Result: Accepted Time: 12 ms

Here should be some explanations.

class Solution {
public:
vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {
sort(nums1.begin(),nums1.end());
sort(nums2.begin(),nums2.end());
const int alen = nums1.size(), blen = nums2.size();
int a = 0, b = 0;
vector<int> ret;
while(a < alen && b < blen)
{
if( nums1[a] > nums2[b])
b++;
else if(nums1[a] < nums2[b])
a++;
else
ret.push_back(nums1[a]),a++,b++;
}
return ret;
}
};


Complexity Analytics

• Time Complexity: $O(nlog(n))$
• Space Complexity: $O(n)$