Feb 22

Author: siddydutta

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

@@ -25,7 +25,7 @@
 | February 19 | [1415. The k-th Lexicographical String of All Happy Strings of Length n](https://leetcode.com/problems/the-k-th-lexicographical-string-of-all-happy-strings-of-length-n/) | Medium | Unsolved |
 | 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 | []() |  |  |
+| February 22 | [1028. Recover a Tree From Preorder Traversal](https://leetcode.com/problems/recover-a-tree-from-preorder-traversal/) | Hard | Solved |
 | February 23 | []() |  |  |
 | February 24 | []() |  |  |
 | February 25 | []() |  |  |
@@ -39,4 +39,4 @@
 | ---    | --- | --- | --- |
 | Easy   | 6 | 6 | 0 |
 | Medium | 14 | 11 | 3 |
-| Hard   | 0 | 0 | 0 |
+| Hard   | 1 | 1 | 0 |

2025-02-February-LeetCoding-Challenge/Recover a Tree From Preorder Traversal.py

@@ -0,0 +1,66 @@
+from typing import 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 recoverFromPreorder(self, traversal: str) -> Optional[TreeNode]:
+        tree = {-1: TreeNode()}  # dummy
+        depth, value = 0, ''
+        for ch in traversal + '-':
+            if ch == '-' and value:
+                parent = tree[depth - 1]
+                child = TreeNode(int(value))
+                if parent.left is None:
+                    parent.left = child
+                else:
+                    parent.right = child
+                tree[depth] = child
+                depth = 1
+                value = ''
+            elif ch == '-':
+                depth += 1
+            else:
+                value += ch
+        return tree[-1].left
+
+
+def main():
+    traversal = '1-2--3--4-5--6--7'
+    root = Solution().recoverFromPreorder(traversal)
+    assert root.val == 1
+    assert root.left.val == 2
+    assert root.right.val == 5
+    assert root.left.left.val == 3
+    assert root.left.right.val == 4
+    assert root.right.left.val == 6
+    assert root.right.right.val == 7
+
+    traversal = '1-2--3---4-5--6---7'
+    root = Solution().recoverFromPreorder(traversal)
+    assert root.val == 1
+    assert root.left.val == 2
+    assert root.right.val == 5
+    assert root.left.left.val == 3
+    assert root.left.left.left.val == 4
+    assert root.right.left.val == 6
+    assert root.right.left.left.val == 7
+
+    traversal = '1-401--349---90--88'
+    root = Solution().recoverFromPreorder(traversal)
+    assert root.val == 1
+    assert root.left.val == 401
+    assert root.right is None
+    assert root.left.left.val == 349
+    assert root.left.right.val == 88
+    assert root.left.left.left.val == 90
+
+
+if __name__ == '__main__':
+    main()