A classic find the needle in the haystack problem. The task is to find if a string is contained within another string and return the index of the first position. If the string is not contained return -1 and if the needle is empty, return 0.
class Solution:
def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
root = ListNode()
current = root
while list1 is not None and list2 is not None:
if list1.val < list2.val:
current.next = ListNode(list1.val)
current = current.next
list1 = list1.next
else:
current.next = ListNode(list2.val)
current = current.next
list2 = list2.next
if list1 is not None:
current.next = list1
if list2 is not None:
current.next = list2
return root.next
A subset of a list with the elements {a,b,c} is {a,b}. The task is to find all possible subsets. Sets in programming languages are usually not ordered. Therefore, the set {a,b,c} is equivalent to {b,c,a}.
The idea for the solution is a nested for-loop that iterates over the results again and again, starting with one empty ([]) result.
The goal of this task is to find all possible permutations of numbers in a list. For example, [1,2,3] can become [[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], [3,2,1]. Mathematically, the number of possible permutations without using duplicates is n! where n is the number of unique elements in the list.
This problem can best be solved by using backtracing and recursion. The general idea is to put all possible digits in the beginning and then cut off the rest of the list and repeat the first step. Later, the list is assembled. The algorithm stops if there is only one item in the list because the only possible permutation is with itself. The other special case that needs to be handled is the empty list which must return the empty list itself.
Great tutorial how to use Devise with Rails 7 and Turbo: Youtube.