t2

post-in.py

@@ -0,0 +1,31 @@
+# O(n) time and space
+# same as pre order in order but reverse pointer and right and left calls
+# 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 buildTree(self, inorder: List[int], postorder: List[int]) -> Optional[TreeNode]:
+        self.idx = len(postorder)-1
+        # Create an empty dictionary
+        inorder_map = {}
+
+        # Use a standard loop with enumerate to fill it: { val: i }
+        for i, val in enumerate(inorder):
+            inorder_map[val] = i
+        return self.helper(postorder, 0, len(inorder) - 1, inorder_map)
+
+    def helper(self, postorder, start, end, inorder_map):
+        if start > end:
+            return None 
+        # move preorder index by 1 each time
+        rootVal = postorder[self.idx]
+        self.idx-=1
+        root = TreeNode(rootVal)
+        rootIdx = inorder_map[rootVal]
+        root.right = self.helper(postorder, rootIdx+1,end,inorder_map)
+        root.left = self.helper(postorder,start,rootIdx-1,inorder_map)
+
+        return root 

root-leaf.py

@@ -0,0 +1,25 @@
+# O(n) time - we visit every node
+# O(h) space - recursion stack is height of tree
+# 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 __init__(self):
+    def sumNumbers(self, root: Optional[TreeNode]) -> int:
+        self.result=0
+        self.helper(root,0)
+        return self.result 
+
+    def helper(self,root, curr):
+        if root == None:
+            return
+
+        curr = curr*10+root.val
+        if root.left is None and root.right is None:
+            self.result+=curr
+
+        self.helper(root.left,curr)
+        self.helper(root.right,curr)