add 21.py

Author: BrianRuizy

images/21 - Merge Two Sorted Lists.md renamed to 21 - Merge Two Sorted Lists.md

@@ -0,0 +1,46 @@
+from typing import Optional
+
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+
+class Solution:
+    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
+        
+        # create sentinel node
+        s = ListNode(0, None)
+        previous = s
+        
+        while list1 is not None and list2 is not None:
+            # compare value at l1 vs value at l2
+            # whichever is smaller that's where we point previous, and step that list
+            # essentially prev keeps track of the new list
+            
+            if list1.val <= list2.val:
+                previous.next = list1
+                list1 = list1.next
+            else: 
+                previous.next = list2
+                list2 = list2.next
+                
+            previous = previous.next
+        
+        # the while loop only runs while l1 or lx2 are not None
+        
+        # since lists can be diffent sizes,
+        # when we've reached the end for one we can point
+        # the merged list to the non-null list
+        previous.next = list1 or list2
+        
+        return s.next
+
+
+
+l1 = ListNode(val=1, next=ListNode(val=2, next=ListNode(val=3, next=None)))
+l2 = ListNode(val=4, next=ListNode(val=5, next=None))
+
+s = Solution()
+s.mergeTwoLists(list1=l1, list2=l2)

21.py

@@ -0,0 +1,32 @@
+# Definition for a binary tree node.
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+
+
+class Solution:
+    def invertTree(self, root) -> Optional[TreeNode]:
+        # do if root is not null
+        if root:
+            self.invertTree(root.left)
+            self.invertTree(root.right)
+            
+            root.left, root.right = root.right, root.left
+        
+        return root
+
+ 
+if '__name__' == '__main__':
+    root = TreeNode(1)
+    root.left = TreeNode(2)
+    root.right = TreeNode(3)
+    root.left.left = TreeNode(4)
+    root.left.right = TreeNode(5)
+    root.right.left = TreeNode(6)
+    root.right.right = TreeNode(7)
+    
+    sol = Solution()
+    sol.invertTree(root)