answers for double_linked_list practice probs

Author: ellieko

data_structures/3_LinkedList/double_linked_list.py

@@ -0,0 +1,161 @@
+class Node:
+    def __init__(self, data=None, next=None, prev=None):
+        self.data = data
+        self.next = next
+        self.prev = prev
+
+
+class DoubleLinkedList:
+    def __init__(self):
+        self.head = None
+        self.tail = None
+
+    def isHead(self, node):
+        return(node == self.head)
+
+    def isTail(self, node):
+        return(node == self.tail)
+
+    def print_forward(self):
+        # This method prints list in forward direction. Use node.next
+        if self.head is None:
+            print("Double linked list is empty")
+            return
+        itr = self.head
+        llstr = ''
+        while itr:
+            llstr += str(itr.data) + ' --> ' if itr.next else str(itr.data)
+            itr = itr.next
+        print(llstr)
+
+    def print_backward(self):
+        # Print linked list in reverse direction. Use node.prev for this.
+        if self.tail is None:
+            print("Double linked list is empty")
+            return
+        itr = self.tail
+        llstr = ''
+        while itr:
+            llstr += str(itr.data) + ' <-- ' if itr.prev else str(itr.data)
+            itr = itr.prev
+        print(llstr)
+
+    def get_length(self):
+        count = 0
+        itr = self.head
+        while itr:
+            count += 1
+            itr = itr.next
+        return count
+
+    def insert_at_beginning(self, data):
+        if self.get_length() == 0:
+            node = Node(data)
+            self.head = node
+            self.tail = node
+        else:
+            old_head = self.head
+            node = Node(data, old_head)
+            self.head = node
+            old_head.prev = node
+
+    def insert_at_end(self, data):
+        if self.get_length() == 0:
+            self.insert_at_beginning(data)
+        else:
+            old_tail = self.tail
+            node = Node(data, None, old_tail)
+            self.tail = node
+            old_tail.next = node
+
+    def insert_at(self, index, data):
+        if index < 0 or index >= self.get_length():
+            raise Exception("Invalid Index")
+        if index == 0:
+            self.insert_at_beginning(data)
+        itr = self.head
+        count = 0
+        while itr:
+            if count == index - 1:
+                next_node = itr.next
+                node = Node(data, next_node, itr)
+                itr.next = node
+                next_node.prev = node
+                break
+            count += 1
+            itr = itr.next
+
+    def remove_at(self, index):
+        # index doesn't match
+        if index < 0 or index >= self.get_length():
+            raise Exception("Invalid Index")
+        # remove head
+        if index == 0:
+            if self.isTail(self.head):
+                self.tail = None
+                self.head = None
+            else:
+                self.head = self.head.next
+                self.head.prev = None
+            return
+        # remove head
+        if index == self.get_length() - 1:
+            self.tail = self.tail.prev
+            self.tail.next = None
+            return
+        # other cases
+        itr = self.head
+        count = 0
+        while itr.next:
+            if count == index - 1:
+                if self.isTail(itr.next):
+                    itr.next.next = None
+                else:
+                    itr.next.next.prev = itr
+                    itr.next = itr.next.next
+                break
+            itr = itr.next
+            count += 1
+
+    def insert_values(self, data_list):
+        self.head = None
+        self.tail = None
+        for data in data_list:
+            self.insert_at_end(data)
+
+    def insert_after_value(self, data_after, data_to_insert):
+        itr = self.head
+        while itr:
+            if itr.data == data_after:
+                node = Node(data_to_insert, itr.next, itr)
+                itr.next.prev = node
+                itr.next = node
+                break
+            itr = itr.next
+
+    def remove_by_value(self, data):
+        itr = self.head
+        while itr:
+            if itr.data == data:
+                if self.isHead(itr):
+                    if self.isTail(itr):
+                        self.head = None
+                        self.tail = None
+                    else:
+                        self.head = itr.next
+                        self.head.prev = None
+                elif self.isTail(itr):
+                    self.tail = self.tail.prev
+                    self.tail.next = None
+                else:
+                    itr.prev.next = itr.next
+                    itr.next.prev = itr.prev
+                break
+            itr = itr.next
+
+
+if __name__ == '__main__':
+    ll = DoubleLinkedList()
+    ll.insert_values([45, 7, 12, 567, 99])
+    ll.remove_by_value(99)
+    ll.print_forward()