Leaves - Emily V

lib/linked_list.rb

@@ -1,45 +1,225 @@
-require_relative 'node'
 
-class LinkedList
-  attr_reader :head
+# Defines a node in the singly linked list
+class Node
+  attr_reader :data # allow external entities to read value but not write
+  attr_accessor :next # allow external entities to read or write next node
+
+  def initialize(value, next_node = nil)
+    @data = value
+    @next = next_node
+  end
+end
 
+# Defines the singly linked list
+class LinkedList
   def initialize
-    @head = nil
+    @head = nil # keep the head private. Not accessible outside this class
   end
-
-  # Time complexity - ?
-  # Space complexity - ?
-  def add_first(data)
-
+  
+  # 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
+  def add_first(value)
+    @head = Node.new(value, @head)
   end
-
-  # Time complexity - ?
-  # Space complexity - ?
-  def get_first
-
+
+  # method to find if the linked list contains a node with specified value
+  # returns true if found, false otherwise
+  def search(value)
+    current = @head
+    until current.nil?
+      return true if current.data == value
+      current = current.next
+    end 
+    return false 
   end
-
-  # Time complexity - ?
-  # Space complexity - ?
+
+  # method to return the max value in the linked list
+  # returns the data value and not the node
+  def find_max
+    return nil if @head.nil?
+    max_data = @head.data
+    curr = @head
+    until curr == nil
+      max_data = curr.data if curr.data > max_data
+      curr = curr.next
+    end 
+    return max_data
+  end
+
+  # method to return the min value in the linked list
+  # returns the data value and not the node
+  def find_min
+    return nil if @head.nil?
+    min_data = @head.data
+    curr = @head
+    until curr == nil
+      min_data = curr.data if curr.data < min_data
+      curr = curr.next
+    end 
+    return min_data
+  end
+
+
+  # method that returns the length of the singly linked list
   def length
-    return 0
+    length = 0
+    curr = @head
+    until curr.nil?
+      curr = curr.next
+      length += 1
+    end 
+    return length
   end
-
-  # Time complexity - ?
-  # Space complexity - ?
-  def add_last(data)
-
+
+  # method that returns the value at a given index in the linked list
+  # index count starts at 0
+  # returns nil if there are fewer nodes in the linked list than the index value
+  def get_at_index(index)
+    curr = @head
+    index.times do 
+      curr.nil? ? return : curr = curr.next
+    end 
+    return curr.data
   end
-
-  # Time complexity - ?
-  # Space complexity - ?
+
+  # method to print all the values in the linked list
+  def visit
+    curr = @head
+    until curr.nil?
+      puts curr.data
+      curr = curr.next
+    end 
+  end
+
+  # method to delete the first node found with specified value
+  def delete(value)
+    curr = prev = @head
+    return if @head.nil? 
+    @head = @head.next if @head.data == value 
+    until curr.nil?
+      if curr.data == value
+        prev.next = curr.next
+        return 
+      end 
+      prev = curr
+      curr = curr.next
+    end 
+  end
+
+  # method to reverse the singly linked list
+  # note: the nodes should be moved and not just the values in the nodes
+  def reverse
+    curr = @head
+    prev = nil
+    until curr.nil?
+      next_node = curr.next
+      curr.next = prev
+      prev = curr
+      curr = next_node
+    end 
+    @head = prev
+  end
+
+
+  ## Advanced Exercises
+  # returns the value at the middle element in the singly linked list
+  def find_middle_value
+    fast = slow = @head
+    until fast.next.nil?
+      2.times do 
+        fast = fast.next if !fast.nil?
+      end
+      slow = slow.next
+    end 
+  end
+
+  # find the nth node from the end and return its value
+  # assume indexing starts at 0 while counting to n
+  def find_nth_from_end(n)
+    return if @head.nil?
+    k_ahead = curr = @head
+    n.times do 
+      k_ahead = k_ahead.next 
+      return if k_ahead.nil?
+    end 
+    until k_ahead.next.nil?
+      k_ahead = k_ahead.next
+      curr = curr.next
+    end 
+    return curr.data
+  end
+
+  # checks if the linked list has a cycle. A cycle exists if any node in the
+  # linked list links to a node already visited.
+  # returns true if a cycle is found, false otherwise.
+  def has_cycle
+    nodes_visited = {}
+    curr = @head
+    until curr.nil?
+      if nodes_visited[curr]
+        return true
+      else 
+        nodes_visited[curr] = true
+      end 
+    end 
+    return false
+  end
+
+
+  # Additional Exercises 
+  # returns the value in the first node
+  # returns nil if the list is empty
+  def get_first
+    return @head.nil? ? nil : @head.data
+  end
+
+  # method that inserts a given value as a new last node in the linked list
+  def add_last(value)
+    curr = @head
+    if curr.nil?
+      @head = Node.new(value, nil) 
+      return
+    end 
+    until curr.next == nil
+      curr = curr.next
+    end 
+    curr.next = Node.new(value, nil)
+  end
+
+  # method that returns the value of the last node in the linked list
+  # returns nil if the linked list is empty
   def get_last
-
+    return if @head.nil?
+    curr = @head
+    until curr.next.nil?
+      curr = curr.next
+    end 
+    return curr.data
   end
-
-  # Time complexity - ?
-  # Space complexity - ?
-  def get_at_index(index)
-
+
+  # method to insert a new node with specific data value, assuming the linked
+  # list is sorted in ascending order
+  def insert_ascending(value)
+    curr = @head
+    while curr.data < value
+      prev = curr
+      curr = curr.next
+    end 
+    prev.next = Node.new(value, curr)
+  end
+
+  # Helper method for tests
+  # Creates a cycle in the linked list for testing purposes
+  # Assumes the linked list has at least one node
+  def create_cycle
+    return if @head == nil # don't do anything if the linked list is empty
+
+    # navigate to last node
+    current = @head
+    while current.next != nil
+      current = current.next
+    end
+
+    current.next = @head # make the last node link to first node
   end
 end

test/linked_list_test.rb

@@ -12,50 +12,50 @@
   before do
     @list = LinkedList.new
   end
-
+  
   describe 'initialize' do
     it 'can be created' do
-
+      
       # Assert
       expect(@list).must_be_kind_of LinkedList
     end
   end
-
-  xdescribe 'add_first & get_first' do
+  
+  describe 'add_first & get_first' do
     it 'can add values to an empty list' do
       # Act
-        @list.add_first(3)
-
-        # Assert
-        expect(@list.get_first).must_equal 3
+      @list.add_first(3)
+      
+      # Assert
+      expect(@list.get_first).must_equal 3
     end
-
+    
     it 'will put the last added item to the front of the list' do
       # Act
       @list.add_first(1)
       @list.add_first(2)
-
+      
       # Assert
       expect(@list.get_first).must_equal 2
-
+      
       # Act again
       @list.add_first(3)
-
+      
       # Assert
       expect(@list.get_first).must_equal 3
     end
-
+    
     it 'will return `nil` for `getFirst` if the list is empty' do
-
+      
       expect(@list.get_first).must_be_nil
     end
   end
-
-  xdescribe "length" do
+  
+  describe "length" do
     it "will return 0 for an empty list" do
       expect(@list.length).must_equal 0
     end
-
+    
     it "will return the length for nonempty lists" do
       count = 0
       while count < 5
@@ -65,43 +65,43 @@
       end
     end
   end
-
-  xdescribe "addLast & getLast" do
+  
+  describe "addLast & getLast" do
     it "will add to the front if the list is empty" do
       @list.add_last(1)
       expect(@list.get_first).must_equal 1
     end
-
+    
     it "will put new items to the rear of the list" do
       @list.add_last(2)
       expect(@list.length).must_equal 1
       expect(@list.get_last).must_equal 2
-
+      
       @list.add_last(3)
       expect(@list.get_first).must_equal 2
       expect(@list.get_last).must_equal 3
       expect(@list.length).must_equal 2
-
+      
       @list.add_last(4)
       expect(@list.get_first).must_equal 2
       expect(@list.get_last).must_equal 4
       expect(@list.length).must_equal 3
-
+      
     end
-
+    
   end
-
-  xdescribe 'get_at_index' do
+  
+  describe 'get_at_index' do
     it 'returns nil if the index is outside the bounds of the list' do
       expect(@list.get_at_index(3)).must_be_nil
     end
-
+    
     it 'can retrieve an item at an index in the list' do
       @list.add_first(1)
       @list.add_first(2)
       @list.add_first(3)
       @list.add_first(4)
-
+      
       expect(@list.get_at_index(0)).must_equal 4
       expect(@list.get_at_index(1)).must_equal 3
       expect(@list.get_at_index(2)).must_equal 2