diff --git a/linked_list/linked_list.py b/linked_list/linked_list.py index 63993214..1c8b0890 100644 --- a/linked_list/linked_list.py +++ b/linked_list/linked_list.py @@ -1,5 +1,6 @@ # Defines a node in the singly linked list + class Node: def __init__(self, value, next_node = None): @@ -7,73 +8,166 @@ def __init__(self, value, next_node = None): self.next = next_node # Defines the singly linked list +# Starting off a linked list is always constant time and space complexity. class LinkedList: def __init__(self): - self.head = None # keep the head private. Not accessible outside this class + self.head = None # keep the head private. Not accessible outside this class # returns the value in the first node # returns None if the list is empty - # Time Complexity: ? - # Space Complexity: ? + # Time Complexity: O(1) + # Space Complexity: O(1) def get_first(self): - pass + if self.head is None: + return None + + return self.head.value # method to add a new node with the specific data value in the linked list # insert the new node at the beginning of the linked list - # Time Complexity: ? - # Space Complexity: ? + # Time Complexity: O(1) + # Space Complexity: O(1) def add_first(self, value): - pass + new_node = Node(value) + new_node.next = self.head + self.head = new_node + + # more compact, not as readable, need to look at the constructor to understand + # self.head references the original linked list that the new head is assigned to + # self.head = Node(value, self.head) + # method to find if the linked list contains a node with specified value # returns true if found, false otherwise - # Time Complexity: ? - # Space Complexity: ? + # Time Complexity: O(n) + # Space Complexity: O(1) def search(self, value): - pass + # initialize a node pointer + current = self.head + + while current != None: + if current.value == value: + return True # Value was found + + current = current.next + + return False # Value not found # method that returns the length of the singly linked list - # Time Complexity: ? - # Space Complexity: ? + # Time Complexity: O(n) + # Space Complexity: O(1) def length(self): - pass + pointer = self.head + counter = 0 + + while pointer != None: + counter += 1 + pointer = pointer.next + + return counter # method that returns the value at a given index in the linked list # index count starts at 0 # returns None if there are fewer nodes in the linked list than the index value - # Time Complexity: ? - # Space Complexity: ? + # Time Complexity: O(n) + # Space Complexity: 0(1) def get_at_index(self, index): - pass + if index < 0: + return None + + current_index = 0 + pointer = self.head + + while pointer != None: + if current_index == index: + return pointer.value + pointer = pointer.next + current_index += 1 # method that returns the value of the last node in the linked list # returns None if the linked list is empty - # Time Complexity: ? - # Space Complexity: ? + # Time Complexity: O(n) + # Space Complexity: O(1) def get_last(self): - pass + # pointer = self.head + # next_node = pointer.next + + # while next_node != None: + # pointer = next_node + # next_node = pointer.next + + if self.head is None: + return None + + pointer = self.head + + while pointer.next is not None: + pointer = pointer.next + + return pointer.value # method that inserts a given value as a new last node in the linked list - # Time Complexity: ? - # Space Complexity: ? + # Time Complexity: O(n) + # Space Complexity: O(1) def add_last(self, value): - pass + new_node = Node(value) + + if self.head is None: + self.head = new_node + return + + last = self.head + while last.next: + last = last.next + + last.next = new_node + # method to return the max value in the linked list # returns the data value and not the node def find_max(self): - pass + if self.head is None: + return None + + pointer = self.head + maximum = self.head.value + + while pointer is not None: + if maximum < pointer.value: + maximum = pointer.value + pointer = pointer.next + + return maximum + # method to delete the first node found with specified value - # Time Complexity: ? - # Space Complexity: ? + # Time Complexity: O(n) + # Space Complexity: O(1) def delete(self, value): - pass + position = self.head + + if position is not None: + if position.value == value: + self.head = position.next + position = None + return + + while position is not None: + if position.value == value: + break + prev = position + position = position.next + + if position == None: + return + + prev.next = position.next + position = None # method to print all the values in the linked list - # Time Complexity: ? - # Space Complexity: ? + # Time Complexity: O(n) + # Space Complexity: O(n) def visit(self): helper_list = [] current = self.head @@ -86,11 +180,24 @@ def visit(self): # method to reverse the singly linked list # note: the nodes should be moved and not just the values in the nodes - # Time Complexity: ? - # Space Complexity: ? + # Time Complexity: O(n) + # Space Complexity: O(1) def reverse(self): - pass - + prev = None + current = self.head + next_node = current.next + + while current: + current.next = prev + prev = current + current = next_node + if next_node: + next_node = next_node.next + + self.head = prev + +# ====================================================================================== + ## Advanced/ Exercises # returns the value at the middle element in the singly linked list # Time Complexity: ?