add a solution and update doc

Author: sangheestyle

Python/copy-list-with-random-pointer.py

@@ -58,4 +58,25 @@ def copyRandomList(self, head):
 
         return dummy.next
 
+# time: O(n)
+# space: O(n)
+from collections import defaultdict
 
+
+class Solution3(object):
+    def copyRandomList(self, head):
+        """
+        :type head: RandomListNode
+        :rtype: RandomListNode
+        """
+        clone = defaultdict(lambda: RandomListNode(0))
+        clone[None] = None
+        cur = head
+
+        while cur:
+            clone[cur].label = cur.label
+            clone[cur].next = clone[cur.next]
+            clone[cur].random = clone[cur.random]
+            cur = cur.next
+
+        return clone[head]

README.md

@@ -292,7 +292,7 @@
 082| [Remove Duplicates from Sorted List II](https://leetcode.com/problems/remove-duplicates-from-sorted-list-ii/)| [C++](./C++/remove-duplicates-from-sorted-list-ii.cpp) [Python](./Python/remove-duplicates-from-sorted-list-ii.py) | _O(n)_       | _O(1)_         | Medium         ||
 083| [Remove Duplicates from Sorted List](https://leetcode.com/problems/remove-duplicates-from-sorted-list/)| [C++](./C++/remove-duplicates-from-sorted-list.cpp) [Python](./Python/remove-duplicates-from-sorted-list.py) | _O(n)_       | _O(1)_         | Easy           ||
 092| [Reverse Linked List II](https://leetcode.com/problems/reverse-linked-list-ii/)| [C++](./C++/reverse-linked-list-ii.cpp) [Python](./Python/reverse-linked-list-ii.py) | _O(n)_       | _O(1)_         | Medium         || 
-138| [Copy List with Random Pointer](https://leetcode.com/problems/copy-list-with-random-pointer/) | [C++](./C++/copy-list-with-random-pointer.cpp) [Python](./Python/copy-list-with-random-pointer.py) | _O(n)_   | _O(1)_          | Hard         ||
+138| [Copy List with Random Pointer](https://leetcode.com/problems/copy-list-with-random-pointer/) | [C++](./C++/copy-list-with-random-pointer.cpp) [Python](./Python/copy-list-with-random-pointer.py) | _O(n)_   | _O(1)_          | Medium         ||
 160| [Intersection of Two Linked Lists](https://leetcode.com/problems/intersection-of-two-linked-lists/)| [C++](./C++/intersection-of-two-linked-lists.cpp)  [Python](./Python/intersection-of-two-linked-lists.py) | _O(m + n)_ | _O(1)_         | Easy           ||
 203| [Remove Linked List Elements](https://leetcode.com/problems/remove-linked-list-elements/)| [C++](./C++/remove-linked-list-elements.cpp) [Python](./Python/remove-linked-list-elements.py) | _O(n)_       | _O(1)_         | Easy         || 
 206| [Reverse Linked List](https://leetcode.com/problems/reverse-linked-list/)| [C++](./C++/reverse-linked-list.cpp) [Python](./Python/reverse-linked-list.py) | _O(n)_       | _O(1)_         | Easy         ||