day4: python prob 9

Author: mandliya

README.md

@@ -7,10 +7,10 @@
 | Current Status|     Stats     |
 | :------------: | :----------: |
 | Total C++ Problems | 188 |
-| Total Python Problems | 8 |
-| Current Daily Streak| 3 |
+| Total Python Problems | 9 |
+| Current Daily Streak| 4 |
 | Last Streak | 06/20/2019 - 06/21/2019|
-| Current Streak | 06/23/2019 - 06/25/2019|
+| Current Streak | 06/23/2019 - 06/26/2019|
 
 </center>
 
@@ -22,7 +22,7 @@
 | Find the nth node of linked list from last. |[nthToLastNode.cpp](linked_list_problems/nthToLastNode.cpp), [nth_to_last_node.py](linked_list_problems/nth_to_last_node.py) |
 | Add numbers where each digit of the number is represented by node of a linkedlist.  Give output as a linked list. | [add_two_numbers_lists.cpp](linked_list_problems/add_two_numbers_lists.cpp), [add_two_numbers_list.py](linked_list_problems/add_two_numbers_lists.py)|
 | Swap nodes of a linkedlist without swapping data. |[swapNodesWithoutSwappingData.cpp](linked_list_problems/swapNodesWithoutSwappingData.cpp), [swap_nodes_without_swapping_data.py](linked_list_problems/swap_nodes_without_swapping_data.py)|
-| Reverse a linked list, iteratively and recursively | [reverseLinkedListIterAndRecurse.cpp](linked_list_problems/reverseLinkedListIterAndRecurse.cpp)|
+| Reverse a linked list, iteratively and recursively | [reverseLinkedListIterAndRecurse.cpp](linked_list_problems/reverseLinkedListIterAndRecurse.cpp), [reverse_linkedlist.py](linked_list_problems/reverse_linkedlist.py)|
 | Given a linked list, reverse alternate nodes and append at the end. | [reverseAlternateNodes.cpp](linked_list_problems/reverseAlternateNodes.cpp) |
 | Only given a node pointer, delete the node from the linked list. | [deleteNode.cpp](linked_list_problems/deleteNode.cpp)|
 | Delete the entire linkedlist. | [deleteLinkedlist.cpp](linked_list_problems/deleteLinkedlist.cpp)|

linked_list_problems/reverse_linkedlist.py

@@ -0,0 +1,81 @@
+"""
+Reverse a linked list
+"""
+
+class Node:
+    """Representation of a linked list node"""
+    def __init__(self, data):
+        self.data = data
+        self.next = None
+
+class LinkedList:
+    """Representation of a linked list"""
+    def __init__(self):
+        self.head = None
+
+    def push(self, data):
+        """Pushes a new node with data at the head of the list"""
+        new_node = Node(data)
+        new_node.next = self.head
+        self.head = new_node
+
+    def print(self):
+        """Prints the linked list"""
+        itr = self.head
+        while itr:
+            if itr.next is None:
+                print(itr.data)
+            else:
+                print(str(itr.data), '-->', end=' ')
+            itr = itr.next
+
+    def _reverse_recur(self, node):
+        """Utility function to recursively reverse a list"""
+        if node is None:
+            return
+
+        if node.next is None:
+            self.head = node
+            return 
+    
+        first = node
+        rest = node.next
+        self._reverse_recur(rest)
+        first.next.next = first
+        first.next = None
+
+    def reverse_r(self):
+        """Recursively reverses a list"""
+        self._reverse_recur(self.head)
+
+    def reverse_i(self):
+        """Iteratively reverses a list"""
+        if (self.head is None) or \
+            (self.head and self.head.next is None):
+            return
+
+        prev = None
+        curr = self.head
+        new_head = None
+        while curr:
+            prev = curr
+            curr = curr.next
+            prev.next = new_head
+            new_head = prev
+        self.head = new_head
+            
+
+
+if __name__ == "__main__":
+    ll = LinkedList()
+    ll.push(5)
+    ll.push(4)
+    ll.push(3)
+    ll.push(2)
+    ll.push(1)
+    ll.print()
+
+    ll.reverse_i()
+    ll.print()
+    ll.reverse_r()
+    ll.print()