Create Flatten a Multilevel Doubly Linked List.py

Author: sugia

Flatten a Multilevel Doubly Linked List.py

@@ -0,0 +1,61 @@
+'''
+You are given a doubly linked list which in addition to the next and previous pointers, it could have a child pointer, which may or may not point to a separate doubly linked list. These child lists may have one or more children of their own, and so on, to produce a multilevel data structure, as shown in the example below.
+
+Flatten the list so that all the nodes appear in a single-level, doubly linked list. You are given the head of the first level of the list.
+
+Example:
+
+Input:
+ 1---2---3---4---5---6--NULL
+         |
+         7---8---9---10--NULL
+             |
+             11--12--NULL
+
+Output:
+1-2-3-7-8-11-12-9-10-4-5-6-NULL
+Explanation for the above example:
+
+Given the following multilevel doubly linked list:
+
+
+
+We should return the following flattened doubly linked list:
+
+
+'''
+
+"""
+# Definition for a Node.
+class Node(object):
+    def __init__(self, val, prev, next, child):
+        self.val = val
+        self.prev = prev
+        self.next = next
+        self.child = child
+"""
+class Solution(object):
+    def flatten(self, head):
+        """
+        :type head: Node
+        :rtype: Node
+        """
+        if not head:
+            return None
+        stack = [head]
+        dummy = Node(0, None, None, None)
+        current = dummy
+        while stack:
+            top = stack.pop()
+            current.next = top
+            top.prev = current
+            current = current.next
+            if top.next:
+                stack.append(top.next)
+                top.next = None
+            if top.child:
+                stack.append(top.child)
+                top.child = None
+        
+        dummy.next.prev = None
+        return dummy.next