add 938

Author: kaidi

Tree/105. Construct Binary Tree from Preorder and Inorder Traversal.py

@@ -40,7 +40,7 @@ def buildTree(self, preorder: List[int], inorder: List[int]) -> TreeNode:
 merely let it pop out the first element in every recursion
 so when the code runs to line root.right = self.buildTree(preorder,right_inorder)
 what left of preorder list is exactly the same as above right_preorder
-but do mind, in this way, we MUST put root.left=self.buildtree() BEFORE root.right=self.buildTree()
+but do keep in mind, in this way, we MUST put root.left=self.buildtree() BEFORE root.right=self.buildTree()
 '''
 
 class Solution:

Tree/938. Range Sum of BST.py

@@ -0,0 +1,29 @@
+# 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 rangeSumBST(self, root: Optional[TreeNode], low: int, high: int) -> int:
+        stack = []
+        s=0
+        while root or stack:
+            while root:
+                stack.append(root)
+                if root.val > low:
+                    root=root.left
+                else:
+                    break
+            root = stack.pop()
+            if low <= root.val <= high:
+                s+=root.val
+            if root.val < high:
+                root = root.right
+            else:
+                root=None
+        return s
+
+# Since the tree is already sorted, 
+# we add two if statements to stop the search of sub left/right tree when root.val is out of the boundary

Tree/Tree_all_traversal.py

@@ -1,3 +1,29 @@
+
+#           [1]
+#           / \
+#         [2]  [3]
+#         / \   
+#       [4]  [5]
+# 
+#
+# Depth First Traversals: 
+# (a) Inorder (Left, Root, Right) : 4 2 5 1 3 
+# (b) Preorder (Root, Left, Right) : 1 2 4 5 3 
+# (c) Postorder (Left, Right, Root) : 4 5 2 3 1
+# Breadth-First or Level Order Traversal: 1 2 3 4 5 
+
+# 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
+
+# We prefer iterative than recursion because recursion algorithms have an overhead for procedure calls
+
+
+
+
 # compare two tree is identical
 def identical(s, t):
     if s is None and t is None: