LeetCode Weekly Contest 195 Swift Solutions

Array subsequences feature!

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Photo by Art Lasovsky on Unsplash

This article is about the 4 challenges in the LeetCode Weekly Contest 195. That is

  • 1496 Path Crossing
  • 1497 Check If Array Pairs Are Divisible by k
  • 1498 Number of Subsequences That Satisfy the Given Sum Condition
  • 1499 Max Value of Equation

The solutions assume some knowledge of Big O notation

The Problems

Each problem will be approached in turn, with a solution and also with articles explaining the tools, techniques and theory that will help you solve these problems yourself.

Let us get started!

I have used some knowledge of Strings and Characters in Swift to make this solution a little faster.

Given a String of paths represented as the single characters ‘N, ‘S’, ‘E’ and ‘W’ a path is formed from the origin (0,0) where ‘N, ‘S’, ‘E’ and ‘W’ represent moving up, down, east and west respectively.

I decided to represent each location as a pair (and since there will be comparisons this will need to conform to Hashable resulting in

struct Pair: Hashable {
var x: Int
var y: Int

Now each character is matched to an ASCII number, that is ‘N, ‘S’, ‘E’ and ‘W’ relates to 78, 69, 83 and 87 respectively. Therefore using a Dictionary of these pairs (with the ASCII UInt8 as an equivalent for the character) can be stored using the following definition

var directions: [UInt8: Pair] = [:]

populated with

directions[78] = Pair(x: 0, y: -1)directions[69] = Pair(x: 1, y: 0)directions[83] = Pair(x: 0, y: 1)directions[87] = Pair(x: -1, y: 0)

The logic of the code then becomes traverse the input `String` (say “NES” Nintendo fans) and check to see if each coordinate has previously been visited coordinate using a Set

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which gave the result

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8ms isn’t too bad — but remember that people may have made faster solutions after this has been published!

Split an array of Integer arr into pairs, such that each pair is divisible by k. If the whole array can be split into such pairs, True should be returned (if not False should be returned).

To solve this in Swift, I’ve used a Dictionary as lookups are typically performed in O(1).

So using % in Swift, we traverse the array storing the frequency of ((num % k) + k) % k in the dictionary (using Swift’s natty default for dictionaries). This looks like:

freq[((num % k) + k) % k, default: 0] += 1

With the main part of the equation ((num % k) + k) % k being so as num % k is not sufficient for negative k, so we add k and %k again as in the equation — making this correct for both positive and negative numbers.

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Which gives the following result:

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Here the given sum condition is that the sum of the minimum and maximum element are less or equal than the `target`.

The question tells us that we should return the result modulo 10⁹ + 7, which here I’ve coded as

let MOD = 1000000007.

This heart of the can be calculated with the following code snippet that precomputes 2^n (which is important as n can be large in this question):

var powPreComp = Array(repeating: 0, count: nums.count + 1)
powPreComp[0] = 1
for i in 1..<nums.count {
powPreComp[i] = 2 * powPreComp[i — 1] % MOD

which results in the array powPreComp containing: [1, 2, 4, 8, 16, 32, 64, …]

We don’t care about the order of the elements — only them minimum and maximum within that sub array. Therefore effectively [1 2 3] = [2 3 1] = [ 3 2 1] = [3 1 2] = [2 1 3].

I’ve gone for Swift’s internal sort here

let arr = nums.sorted()

Now we can use a two-pointer solution to find the maxium array length with the answer — and keep moving that left pointer until the pointers cross. The solution is as follows:

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Giving the following result:

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We are given an array of points that represent the coordinates of points on a 2D plane (which is sorted on the x value).

The task is to find the maximum value of yᵢ + yⱼ + |xᵢ — xⱼ| where |xᵢ - xⱼ and 1 <= i < j <= points.length|

So given an array of points (x, y) there will be a set of pairs (1 element or more) where the difference between x₁ and x₂ is less than k — but which of these pairs will have the maximum value yᵢ + yⱼ + |xᵢ — xⱼ|… out of all of the possible values?

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Giving the solution:

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Another set of LeetCode solutions are done, and of course there might be better solutions pubished in the future on their

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