binary tree using traversal lists

data_structures/binary_tree/build_tree_from_traversal.py

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+"""
+Binary Tree Construction from Preorder/Inorder or Postorder/Inorder sequences.
+
+This module allows building a binary tree when given:
+
+- Preorder and inorder sequences
+- Postorder and inorder sequences
+
+Doctest examples are included for verification.
+"""
+
+from __future__ import annotations # for forward references
+from typing import Optional
+
+
+class Node:
+ """Class representing a node in a binary tree."""
+
+ def __init__(self, data: int) -> None:
+ self.data: int = data
+ self.left: Node | None = None
+ self.right: Node | None = None
+ self.parent: Node | None = None
+
+
+def inorder_traversal(root: Node | None, out: list[int]) -> None:
+ """Perform normal inorder traversal and store values in 'out' list."""
+ if root is None:
+ return
+ inorder_traversal(root.left, out)
+ out.append(root.data)
+ inorder_traversal(root.right, out)
+
+
+def _build_tree_from_preorder(
+ preorder: list[int],
+ pre_start: int,
+ pre_end: int,
+ inorder: list[int],
+ in_start: int,
+ in_end: int,
+ in_map: dict[int, int],
+) -> Node | None:
+ """Internal recursive function to build tree from preorder & inorder."""
+ if pre_start > pre_end or in_start > in_end:
+ return None
+
+ root = Node(preorder[pre_start])
+ in_root_index = in_map[root.data]
+ nums_left = in_root_index - in_start
+
+ root.left = _build_tree_from_preorder(
+ preorder,
+ pre_start + 1,
+ pre_start + nums_left,
+ inorder,
+ in_start,
+ in_root_index - 1,
+ in_map,
+ )
+ root.right = _build_tree_from_preorder(
+ preorder,
+ pre_start + nums_left + 1,
+ pre_end,
+ inorder,
+ in_root_index + 1,
+ in_end,
+ in_map,
+ )
+
+ return root
+
+
+def build_tree_from_preorder(inorder: list[int], preorder: list[int]) -> Node | None:
+ """Build binary tree from preorder and inorder sequences."""
+ in_map = {val: idx for idx, val in enumerate(inorder)}
+ return _build_tree_from_preorder(
+ preorder, 0, len(preorder) - 1, inorder, 0, len(inorder) - 1, in_map
+ )
+
+
+def _build_tree_from_postorder(
+ postorder: list[int],
+ post_start: int,
+ post_end: int,
+ inorder: list[int],
+ in_start: int,
+ in_end: int,
+ in_map: dict[int, int],
+) -> Node | None:
+ """
+ Internal recursive function to build tree from postorder & inorder.
+
+ Example:
+ >>> inorder_seq = [1, 2, 3]
+ >>> postorder_seq = [1, 3, 2]
+ >>> inmp = {v:i for i,v in enumerate(inorder_seq)}
+ >>> root = _build_tree_from_postorder(postorder_seq, 0, 2, inorder_seq, 0, 2, inmp)
+ >>> root.data
+ 2
+ >>> root.left.data
+ 1
+ >>> root.right.data
+ 3
+ """
+ if post_start > post_end or in_start > in_end:
+ return None
+
+ root = Node(postorder[post_end])
+ in_root_index = in_map[root.data]
+ nums_left = in_root_index - in_start
+
+ root.left = _build_tree_from_postorder(
+ postorder,
+ post_start,
+ post_start + nums_left - 1,
+ inorder,
+ in_start,
+ in_root_index - 1,
+ in_map,
+ )
+ root.right = _build_tree_from_postorder(
+ postorder,
+ post_start + nums_left,
+ post_end - 1,
+ inorder,
+ in_root_index + 1,
+ in_end,
+ in_map,
+ )
+
+ return root
+
+
+def build_tree_from_postorder(inorder: list[int], postorder: list[int]) -> Node | None:
+ """Build binary tree from postorder and inorder sequences."""
+ in_map = {val: idx for idx, val in enumerate(inorder)}
+ return _build_tree_from_postorder(
+ postorder, 0, len(postorder) - 1, inorder, 0, len(inorder) - 1, in_map
+ )
+
+
+# Optional example usage
+if __name__ == "__main__":
+ inorder_seq = [1, 2, 3, 4, 5]
+ preorder_seq = [3, 2, 1, 4, 5]
+ postorder_seq = [1, 2, 5, 4, 3]
+
+ tree_pre = build_tree_from_preorder(inorder_seq, preorder_seq)
+ tree_post = build_tree_from_postorder(inorder_seq, postorder_seq)
+
+ out: list[int] = []
+ inorder_traversal(tree_pre, out)
+ print("Inorder from Preorder Tree:", out)
+ out.clear()
+ inorder_traversal(tree_post, out)
+ print("Inorder from Postorder Tree:", out)