# Posts published in “Hashtable”

You are given an integer array cards where cards[i] represents the value of the ith card. A pair of cards are matching if the cards have the same value.

Return the minimum number of consecutive cards you have to pick up to have a pair of matching cards among the picked cards. If it is impossible to have matching cards, return -1.

Example 1:

Input: cards = [3,4,2,3,4,7]
Output: 4
Explanation: We can pick up the cards [3,4,2,3] which contain a matching pair of cards with value 3. Note that picking up the cards [4,2,3,4] is also optimal.


Example 2:

Input: cards = [1,0,5,3]
Output: -1
Explanation: There is no way to pick up a set of consecutive cards that contain a pair of matching cards.


Constraints:

• 1 <= cards.length <= 105
• 0 <= cards[i] <= 106

## Solution: Hashtable

Record the last position of each number,
ans = min{cardi – last[cardi]}

Time complexity: O(n)
Space complexity: O(n)

## C++

Given a 2D integer array nums where nums[i] is a non-empty array of distinct positive integers, return the list of integers that are present in each array ofnums sorted in ascending order.

Example 1:

Input: nums = [[3,1,2,4,5],[1,2,3,4],[3,4,5,6]]
Output: [3,4]
Explanation:
The only integers present in each of nums = [3,1,2,4,5], nums = [1,2,3,4], and nums = [3,4,5,6] are 3 and 4, so we return [3,4].

Example 2:

Input: nums = [[1,2,3],[4,5,6]]
Output: []
Explanation:
There does not exist any integer present both in nums and nums, so we return an empty list [].


Constraints:

• 1 <= nums.length <= 1000
• 1 <= sum(nums[i].length) <= 1000
• 1 <= nums[i][j] <= 1000
• All the values of nums[i] are unique.

## Solution: Hashtable

Time complexity: O(n)
Space complexity: O(n)

## C++

You are given an integer array matches where matches[i] = [winneri, loseri] indicates that the player winneri defeated player loseri in a match.

Return a list answer of size 2 where:

• answer is a list of all players that have not lost any matches.
• answer is a list of all players that have lost exactly one match.

The values in the two lists should be returned in increasing order.

Note:

• You should only consider the players that have played at least one match.
• The testcases will be generated such that no two matches will have the same outcome.

Example 1:

Input: matches = [[1,3],[2,3],[3,6],[5,6],[5,7],[4,5],[4,8],[4,9],[10,4],[10,9]]
Output: [[1,2,10],[4,5,7,8]]
Explanation:
Players 1, 2, and 10 have not lost any matches.
Players 4, 5, 7, and 8 each have lost one match.
Players 3, 6, and 9 each have lost two matches.


Example 2:

Input: matches = [[2,3],[1,3],[5,4],[6,4]]
Output: [[1,2,5,6],[]]
Explanation:
Players 1, 2, 5, and 6 have not lost any matches.
Players 3 and 4 each have lost two matches.


Constraints:

• 1 <= matches.length <= 105
• matches[i].length == 2
• 1 <= winneri, loseri <= 105
• winneri != loseri
• All matches[i] are unique.

## Solution: Hashtable

Use a hashtable to store the number of matches each player has lost. Note, also create an entry for those winners who never lose.

Time complexity: O(m), m = # of matches
Space complexity: O(n), n = # of players

## C++

Given two 0-indexed integer arrays nums1 and nums2, return a list answer of size 2 where:

• answer is a list of all distinct integers in nums1 which are not present in nums2.
• answer is a list of all distinct integers in nums2 which are not present in nums1.

Note that the integers in the lists may be returned in any order.

Example 1:

Input: nums1 = [1,2,3], nums2 = [2,4,6]
Output: [[1,3],[4,6]]
Explanation:
For nums1, nums1 = 2 is present at index 0 of nums2, whereas nums1 = 1 and nums1 = 3 are not present in nums2. Therefore, answer = [1,3].
For nums2, nums2 = 2 is present at index 1 of nums1, whereas nums2 = 4 and nums2 = 6 are not present in nums2. Therefore, answer = [4,6].

Example 2:

Input: nums1 = [1,2,3,3], nums2 = [1,1,2,2]
Output: [,[]]
Explanation:
For nums1, nums1 and nums1 are not present in nums2. Since nums1 == nums1, their value is only included once and answer = .
Every integer in nums2 is present in nums1. Therefore, answer = [].


Constraints:

• 1 <= nums1.length, nums2.length <= 1000
• -1000 <= nums1[i], nums2[i] <= 1000

## Solution: Hashtable

Use two hashtables to store the unique numbers of array1 and array2 respectfully.

Time complexity: O(m+n)
Space complexity: O(m+n)

## C++

You are given an integer array nums consisting of 2 * n integers.

You need to divide nums into n pairs such that:

• Each element belongs to exactly one pair.
• The elements present in a pair are equal.

Return true if nums can be divided into n pairs, otherwise return false.

Example 1:

Input: nums = [3,2,3,2,2,2]
Output: true
Explanation:
There are 6 elements in nums, so they should be divided into 6 / 2 = 3 pairs.
If nums is divided into the pairs (2, 2), (3, 3), and (2, 2), it will satisfy all the conditions.


Example 2:

Input: nums = [1,2,3,4]
Output: false
Explanation:
There is no way to divide nums into 4 / 2 = 2 pairs such that the pairs satisfy every condition.


Constraints:

• nums.length == 2 * n
• 1 <= n <= 500
• 1 <= nums[i] <= 500

## Solution: Hashtable

Each number has to appear even numbers in order to be paired. Count the frequency of each number, return true if all of them are even numbers, return false otherwise.

Time complexity: O(n)
Space complexity: O(n)