Friedrich Ewald · My Personal Website

Leetcode: Sort colors

Oct 9, 2022 · 2 min read

Given an array nums with n colored objects, sort the array in place so that the colors are orders. The colors are represented by the numbers 0, 1, and 2. The easiest way to achieve this is via quicksort in O(n*log n).

Leetcode: Find first and last position of element in sorted arrays

Oct 8, 2022 · 7 min read

Given a list of ascending sorted numbers, find the first and last occurrence of a target number. For example for the list [1,2,2,3,4] and target = 2, the result would be [1,2]. If the target number is not in the array, return [-1,-1].

Leetcode: Three sum

Oct 7, 2022 · 4 min read

Given a list of integers, add three integers at the same time so that the sum of those integers is 0. There should be no repetition of integers, for example [-1, 0, 1] is equivalent to [-1, 1, 0].

Leetcode: Longest Consecutive Sequence

Oct 6, 2022 · 3 min read

From an unsorted array nums, return the length of the longest consecutive sequence of numbers as an integer. For example, the list [100, 4, 200, 1, 3, 2] would return 4, because the longest consecutive sequence is [1, 2, 3, 4] with a length of 4. The algorithm should perform in O(n). Not mentioned in the task is that there can be duplicates of numbers. They should be ignored. The main problem when developing this algorithm is for it to perform it in O(n). A brute force method is quite easy and works for smaller lists. The pseudo code looks like this:

for every element in list
  while next element in list
    take next element
    increment counter by 1
  endwhile
  return longest counter
endfor

This approach works but is very time consuming with a complexity of O(n^2). This is because for every element in the list we need to possibly visit every other element in the list.

Leetcode: Happy number

Oct 5, 2022 · 2 min read

Write an algorithm to determine if a number n is happy. A happy number is a number defined by the following process: Starting with any positive integer, replace the number by the sum of the squares of its digits. Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1. Those numbers for which this process ends in 1 are happy. Return true if n is a happy number, and false if not.

For example, the number 19 is divided into 1^2 and 9^2, then added together as 82, then divided again, etc. After some iterations this reaches 100 which is 1^2 + 0^2 + 0^2 = 1. On the other hand, 2 keeps ever incremententing. The programming aspect is relatively easy to solve in Python when converting the types to a string and then converting the string to a list.