Feb 23

Author: siddydutta

2025-02-February-LeetCoding-Challenge/Construct Binary Tree from Preorder and Postorder Traversal.py

@@ -0,0 +1,54 @@
+from typing import List, Optional
+
+
+# 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 constructFromPrePost(
+            self, preorder: List[int], postorder: List[int]
+    ) -> Optional[TreeNode]:
+        root = TreeNode(preorder[0])
+        stack = [root]
+        pre_index, post_index = 1, 0
+        while stack:
+            node = stack[-1]
+            if node.val == postorder[post_index]:
+                stack.pop()
+                post_index += 1
+            else:
+                new_node = TreeNode(preorder[pre_index])
+                pre_index += 1
+                if node.left is None:
+                    node.left = new_node
+                else:
+                    node.right = new_node
+                stack.append(new_node)
+        return root
+
+
+def main():
+    preorder = [1, 2, 4, 5, 3, 6, 7]
+    postorder = [4, 5, 2, 6, 7, 3, 1]
+    root = Solution().constructFromPrePost(preorder, postorder)
+    assert root.val == 1
+    assert root.left.val == 2
+    assert root.right.val == 3
+    assert root.left.left.val == 4
+    assert root.left.right.val == 5
+    assert root.right.left.val == 6
+    assert root.right.right.val == 7
+
+    preorder = [1]
+    postorder = [1]
+    root = Solution().constructFromPrePost(preorder, postorder)
+    assert root.val == 1
+
+
+if __name__ == '__main__':
+    main()

2025-02-February-LeetCoding-Challenge/README.md

@@ -26,7 +26,7 @@
 | February 20 | [1980. Find Unique Binary String](https://leetcode.com/problems/find-unique-binary-string/) | Medium | Solved |
 | February 21 | [1261. Find Elements in a Contaminated Binary Tree](https://leetcode.com/problems/find-elements-in-a-contaminated-binary-tree/) | Medium | Solved |
 | February 22 | [1028. Recover a Tree From Preorder Traversal](https://leetcode.com/problems/recover-a-tree-from-preorder-traversal/) | Hard | Solved |
-| February 23 | []() |  |  |
+| February 23 | [889. Construct Binary Tree from Preorder and Postorder Traversal](https://leetcode.com/problems/construct-binary-tree-from-preorder-and-postorder-traversal/) | Medium | Unsolved |
 | February 24 | []() |  |  |
 | February 25 | []() |  |  |
 | February 26 | []() |  |  |
@@ -38,5 +38,5 @@
 | Level  | Problems | Solved | Unsolved |
 | ---    | --- | --- | --- |
 | Easy   | 6 | 6 | 0 |
-| Medium | 14 | 11 | 3 |
+| Medium | 15 | 12 | 3 |
 | Hard   | 1 | 1 | 0 |