From 97f9cd7826e22ebee1b9e9074e3b019493837466 Mon Sep 17 00:00:00 2001
From: Margaret Finnan <msfinnan@gmail.com>
Date: Sat, 24 Aug 2019 13:30:50 -0700
Subject: [PATCH] finished linked list methods

---
 lib/linked_list.rb | 348 +++++++++++++++++++++++++++++++--------------
 1 file changed, 242 insertions(+), 106 deletions(-)

diff --git a/lib/linked_list.rb b/lib/linked_list.rb
index 501c60fb..6e8affe3 100644
--- a/lib/linked_list.rb
+++ b/lib/linked_list.rb
@@ -6,158 +6,294 @@ class Node
 
   def initialize(value, next_node = nil)
     @data = value
-    @next = next_node
+    @next = nil
   end
 end
 
 # Defines the singly linked list
 class LinkedList
-    def initialize
-      @head = nil # keep the head private. Not accessible outside this class
+  def initialize
+    @head = nil # keep the head private. Not accessible outside this class
+  end
+
+  # 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: Constant
+  # Space Complexity: Constant
+  def add_first(value)
+    new_node = Node.new(value)
+
+    if @head != nil #if list isn't empty
+      new_node.next = @head
     end
 
-    # 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
-    def add_first(value)
-      raise NotImplementedError
+    @head = new_node
+  end
+
+  # method to find if the linked list contains a node with specified value
+  # returns true if found, false otherwise
+  # Time Complexity: o(n) where n is the length of the linked list because this method iterates through each node to find the value
+  # Space Complexity: constant, no extra space needed
+  def search(value)
+    if @head == nil
+      return false
     end
 
-    # method to find if the linked list contains a node with specified value
-    # returns true if found, false otherwise
-    # Time Complexity:  
-    # Space Complexity
-    def search(value)
-      raise NotImplementedError
+    current = @head
+    while @next != nil
+      if current.data == value
+        return true
+      end
     end
+    return false
+  end
+
+  # method to return the max value in the linked list
+  # returns the data value and not the node
+  # Time Complexity: 0(n) where n is length of linked list bc we are checking each node
+  # Space Complexity: constant, extra space needed for variables does not grow as the size of the LL grows
+  def find_max
+    return nil if @head == nil
+
+    current = @head
+    max = current.data
 
-    # method to return the max value in the linked list
-    # returns the data value and not the node
-    # Time Complexity:  
-    # Space Complexity
-    def find_max
-      raise NotImplementedError
+    while current.next != nil
+      if current.data > max
+        max = current.data
+      end
+      current = current.next
     end
 
-    # method to return the min value in the linked list
-    # returns the data value and not the node
-    # Time Complexity:  
-    # Space Complexity
-    def find_min
-      raise NotImplementedError
+    if current.data > max
+      max = current.data
     end
 
+    return max
+    # raise NotImplementedError
+  end
 
-    # method that returns the length of the singly linked list
-    # Time Complexity:  
-    # Space Complexity
-    def length
-      raise NotImplementedError
+  # method to return the min value in the linked list
+  # returns the data value and not the node
+  # Time Complexity: 0(n) where n is length of linked list bc we are checking each node
+  # Space Complexity: constant, extra space needed for variables does not grow as the size of the LL grows
+  def find_min
+    return nil if @head == nil
+    min = @head.data
+    current = @head
+    while current.next != nil
+      if current.data < min
+        min = current.data
+      end
+      current = current.next
     end
 
-    # 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
-    # Time Complexity:  
-    # Space Complexity
-    def get_at_index(index)
-      raise NotImplementedError
+    if current.data < min
+      min = current.data
     end
+    return min
+  end
+
+  # method that returns the length of the singly linked list
+  # Time Complexity: 0(n) where n is length of linked list bc we are checking each node
+  # Space Complexity: constant, extra space needed for variables does not grow as the size of the LL grows
+  def length
+    count = 0
+
+    return count if @head == nil
+
+    current = @head
 
-    # method to print all the values in the linked list
-    # Time Complexity:  
-    # Space Complexity
-    def visit
-      raise NotImplementedError
+    while current.next != nil
+      count += 1
+      current = current.next
     end
 
-    # method to delete the first node found with specified value
-    # Time Complexity:  
-    # Space Complexity
-    def delete(value)
-      raise NotImplementedError
+    return count + 1
+  end
+
+  # 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
+  # Time Complexity: 0(n) where n is length of linked list bc we are checking each node
+  # Space Complexity: constant, extra space needed for variables does not grow as the size of the LL grows
+  def get_at_index(index)
+    return nil if @head == nil
+
+    current = @head
+    count = 0
+
+    until count == index
+      return nil if current.next == nil
+      current = current.next
+      count += 1
     end
+    return current.data
+  end
 
-    # 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
-    def reverse
-      raise NotImplementedError
+  # method to print all the values in the linked list
+  # Time Complexity: 0(n) where n is length of linked list bc we are checking each node
+  # Space Complexity: constant, extra space needed for variables does not grow as the size of the LL grows
+  def visit
+    current = @head
+    while current.next != nil
+      puts current.data
     end
+    puts current.data
+  end
 
+  # method to delete the first node found with specified value
+  # Time Complexity: 0(n) where n is length of linked list bc we are checking each node
+  # Space Complexity: constant, extra space needed for variables does not grow as the size of the LL grows
+  def delete(value)
+    #head is nil
+    return nil if @head == nil
 
-    ## Advanced Exercises
-    # returns the value at the middle element in the singly linked list
-    # Time Complexity:  
-    # Space Complexity
-    def find_middle_value
-      raise NotImplementedError
+    #head is node to delete
+    if @head.data == value
+      @head = @head.next
+      return
     end
 
-    # find the nth node from the end and return its value
-    # assume indexing starts at 0 while counting to n
-    # Time Complexity:  
-    # Space Complexity
-    def find_nth_from_end(n)
-      raise NotImplementedError
+    current = @head
+    while current.next != nil
+      if current.next.data == value
+        current.next = current.next.next
+        return
+      end
+      current = current.next
     end
+  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.
-    # Time Complexity:  
-    # Space Complexity
-    def has_cycle
-      raise NotImplementedError
+  # method to reverse the singly linked list
+  # note: the nodes should be moved and not just the values in the nodes
+  # Time Complexity: 0(n) where n is length of linked list bc we are checking each node
+  # Space Complexity: constant, extra space needed for variables does not grow as the size of the LL grows
+  def reverse
+    current = @head
+    prev = nil
+
+    while current != nil
+      nextNode = current.next
+      current.next = prev
+      prev = current
+      current = nextNode
     end
+    @head = prev
+  end
 
+  ## Advanced Exercises
+  # returns the value at the middle element in the singly linked list
+  # Time Complexity:
+  # Space Complexity
+  def find_middle_value
+    raise NotImplementedError
+  end
 
-    # Additional Exercises 
-    # returns the value in the first node
-    # returns nil if the list is empty
-    # Time Complexity:  
-    # Space Complexity
-    def get_first
-      raise NotImplementedError
+  # find the nth node from the end and return its value
+  # assume indexing starts at 0 while counting to n
+  # Time Complexity:
+  # Space Complexity
+  def find_nth_from_end(n)
+    #copied from solution branch so tests for reverse could run
+    # approach with going through the linked list just once
+    current = @head
+    index = 0
+    # count to n from the beginning
+    while current != nil && index != n
+      current = current.next
+      index += 1
     end
 
-    # method that inserts a given value as a new last node in the linked list
-    # Time Complexity:  
-    # Space Complexity
-    def add_last(value)
-      raise NotImplementedError
+    # check that we didn't reach the end
+    if current == nil
+      # puts "Error: The linked list has less than #{n} indexable nodes"
+      return nil
     end
 
-    # method that returns the value of the last node in the linked list
-    # returns nil if the linked list is empty
-    # Time Complexity:  
-    # Space Complexity
-    def get_last
-      raise NotImplementedError
+    # the previous while loop exited because of index == n condition
+    # start a new traverser at the beginning.
+    # when current reaches the end, new_current will be at index n from the end
+    new_current = @head
+    while current.next != nil
+      current = current.next
+      new_current = new_current.next
     end
+    return new_current.data
+  end
 
-    # method to insert a new node with specific data value, assuming the linked
-    # list is sorted in ascending order
-    # Time Complexity:  
-    # Space Complexity
-    def insert_ascending(value)
-      raise NotImplementedError
+  # 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.
+  # Time Complexity:
+  # Space Complexity
+  def has_cycle
+    raise NotImplementedError
+  end
+
+  # Additional Exercises
+  # returns the value in the first node
+  # returns nil if the list is empty
+  # Time Complexity:Constant, no matter the size of the LL, only have to look at first node
+  # Space Complexity: Constant
+  def get_first
+    if @head == nil
+      return nil
+    else
+      return @head.data
     end
+  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
+  # method that inserts a given value as a new last node in the linked list
+  # Time Complexity: O(n) where n is length of LL bc you have to iterate through entire LL
+  # Space Complexity: Constant
+  def add_last(value)
+    new_node = Node.new(value)
 
-      # navigate to last node
+    if @head == nil
+      @head = new_node
+    else
       current = @head
       while current.next != nil
-          current = current.next
+        current = current.next
       end
+      current.next = new_node
+    end
+  end
+
+  # method that returns the value of the last node in the linked list
+  # returns nil if the linked list is empty
+  # Time Complexity: O(n) because you have to iterate through the entire LL
+  # Space Complexity: Constant
+  def get_last
+    return nil if @head == nil
+    current = @head
+    until current.next == nil
+      current = current.next
+    end
+    return current.data
+  end
 
-      current.next = @head # make the last node link to first node
+  # method to insert a new node with specific data value, assuming the linked
+  # list is sorted in ascending order
+  # Time Complexity:
+  # Space Complexity
+  def insert_ascending(value)
+    raise NotImplementedError
+  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