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55 lines
1.4 KiB
55 lines
1.4 KiB
"""
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File: build_tree.py
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Created Time: 2023-07-15
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Author: krahets (krahets@163.com)
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"""
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import sys
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from pathlib import Path
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sys.path.append(str(Path(__file__).parent.parent))
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from modules import TreeNode, print_tree
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def dfs(
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preorder: list[int],
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inorder_map: dict[int, int],
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i: int,
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l: int,
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r: int,
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) -> TreeNode | None:
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"""Build binary tree: Divide and conquer"""
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# Terminate when subtree interval is empty
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if r - l < 0:
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return None
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# Initialize root node
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root = TreeNode(preorder[i])
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# Query m to divide left and right subtrees
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m = inorder_map[preorder[i]]
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# Subproblem: build left subtree
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root.left = dfs(preorder, inorder_map, i + 1, l, m - 1)
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# Subproblem: build right subtree
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root.right = dfs(preorder, inorder_map, i + 1 + m - l, m + 1, r)
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# Return root node
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return root
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def build_tree(preorder: list[int], inorder: list[int]) -> TreeNode | None:
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"""Build binary tree"""
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# Initialize hash table, storing in-order elements to indices mapping
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inorder_map = {val: i for i, val in enumerate(inorder)}
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root = dfs(preorder, inorder_map, 0, 0, len(inorder) - 1)
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return root
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"""Driver Code"""
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if __name__ == "__main__":
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preorder = [3, 9, 2, 1, 7]
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inorder = [9, 3, 1, 2, 7]
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print(f"Pre-order traversal = {preorder}")
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print(f"In-order traversal = {inorder}")
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root = build_tree(preorder, inorder)
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print("The built binary tree is:")
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print_tree(root)
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