diff --git a/October'21/October_30_Longest_Duplicate_Substring.py b/October'21/October_30_Longest_Duplicate_Substring.py
new file mode 100644
index 0000000..1f12ccf
--- /dev/null
+++ b/October'21/October_30_Longest_Duplicate_Substring.py
@@ -0,0 +1,48 @@
+class Solution:
+    def longestDupSubstring(self, S: str) -> str:
+        l = 1
+        h = len(S)
+        ans = (0,0)
+        while l<=h:
+            mid = l+(h-l)//2
+            pk = self.check(S,mid)
+            if pk:
+                ans = pk
+                l = mid+1
+            else:
+                h = mid-1
+        return S[ans[0]:ans[0]+ans[1]]
+    
+    def check(self, A, k):
+        p = 10**9+7
+        x = [random.randint(1,p-1) for i in range(2)]
+        xFactor = [1]*2
+        for i in range(k):
+            for j in range(2):
+                xFactor[j] = (xFactor[j]*x[j])%p
+        print(x)
+        print(xFactor)
+
+        hashes = {}
+
+        hash = [0,0]
+
+        for i in range(k):
+            for j in range(2):
+                hash[j] = ((hash[j]*x[j])%p +ord(A[i]))%p
+
+        hashes[tuple(hash)] = 0
+
+        for i in range(k,len(A)):
+            for j in range(2):
+                hash[j]=((hash[j]*x[j])%p-(ord(A[i-k])*xFactor[j])%p+ord(A[i]))%p
+            
+            temp = tuple(hash)
+            if temp in hashes:
+                return (hashes[temp],k)
+            hashes[tuple(hash)] = i-k+1
+
+        return None
+
+    
+        
\ No newline at end of file
diff --git a/October'21/October_31_Flatten_a_Multilevel_Doubly_Linked_List.py b/October'21/October_31_Flatten_a_Multilevel_Doubly_Linked_List.py
new file mode 100644
index 0000000..b801447
--- /dev/null
+++ b/October'21/October_31_Flatten_a_Multilevel_Doubly_Linked_List.py
@@ -0,0 +1,34 @@
+"""
+# Definition for a Node.
+class Node:
+    def __init__(self, val, prev, next, child):
+        self.val = val
+        self.prev = prev
+        self.next = next
+        self.child = child
+"""
+
+class Solution:
+    def flatten(self, head: 'Node') -> 'Node':
+        
+        cur = head
+        prev = None
+        parent = []
+        
+        while cur or parent:
+            
+            if cur is None:
+                cur = parent.pop()
+                prev.next = cur
+            
+            if cur.child:
+                if cur.next:
+                    parent.append(cur.next)
+                cur.next = cur.child
+                cur.child = None
+                
+            cur.prev = prev
+            prev = cur
+            cur = cur.next
+                
+        return head
\ No newline at end of file