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Author: Chris Wu

problems/linked-list-cycle-ii.py

@@ -1,3 +1,7 @@
+#Flag
+#Use a flag that stores on the nodes to know if we visited or not.
+#time: O(N).
+#space: O(N), for each node we use O(1) and there are N nodes.
 class Solution(object):
     def detectCycle(self, head):
         curr = head
@@ -7,12 +11,16 @@ def detectCycle(self, head):
             curr = curr.next
         return None
 
+#HashSet
+#Use a hash-set to store the visited nodes
+#time: O(N).
+#space: O(N).
 class Solution(object):
     def detectCycle(self, head):
         visited = set()
         curr = head
         while curr:
-            if curr in visited: return True
+            if curr in visited: return curr
             visited.add(curr)
             curr = curr.next
-        return False
+        return None

problems/merge-two-sorted-lists.py

@@ -1,28 +1,20 @@
 #https://leetcode.com/problems/merge-two-sorted-lists/
+
 class Solution(object):
     def mergeTwoLists(self, l1, l2):
-        if l1==None and l2==None:
-            return None
-        elif l1 and l2==None:
-            return l1
-        elif l1==None and l2:
-            return l2
-        
-        pre = ListNode
-        curr = pre
+        pre_head = ListNode(-1)
+        curr = pre_head
 
         while l1 and l2:
-            if l1.val<=l2.val:
+            if l1.val<l2.val:
                 curr.next = l1
                 l1 = l1.next
             else:
                 curr.next = l2
                 l2 = l2.next
             curr = curr.next
-        
-        if l1:
-            curr.next = l1
-        elif l2:
-            curr.next = l2
-        
-        return pre.next
+            
+        if l1: curr.next = l1
+        if l2: curr.next = l2
+            
+        return pre_head.next