Description
https://leetcode.com/problems/intersection-of-two-arrays-ii/
Given two arrays, write a function to compute their intersection.
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?
Explanation
use a hashmap to check intersections
Python Solution
class Solution:
def intersect(self, nums1: List[int], nums2: List[int]) -> List[int]:
counter = {}
for num in nums1:
if num in counter:
counter[num] += 1
else:
counter[num] = 1
result = []
for num in nums2:
if num in counter and counter[num] != 0:
result.append(num)
counter[num] -= 1
return result
- Time complexity: O(n+m), where n and m are the lengths of the arrays. We iterate through the first, and then through the second array; insert and lookup operations in the hash map take a constant time.
- Space complexity: O(min(n,m)). We use a hash map to store numbers (and their counts) from the smaller array.