You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
55 lines
1.4 KiB
55 lines
1.4 KiB
6 months ago
|
"""
|
||
|
|
File: build_tree.py
|
||
|
|
Created Time: 2023-07-15
|
||
|
|
Author: krahets (krahets@163.com)
|
||
|
|
"""
|
||
|
|
|
||
|
|
import sys
|
||
|
|
from pathlib import Path
|
||
|
|
|
||
|
|
sys.path.append(str(Path(__file__).parent.parent))
|
||
|
|
from modules import TreeNode, print_tree
|
||
|
|
|
||
|
|
|
||
|
|
def dfs(
|
||
|
|
preorder: list[int],
|
||
|
|
inorder_map: dict[int, int],
|
||
|
|
i: int,
|
||
|
|
l: int,
|
||
|
|
r: int,
|
||
|
|
) -> TreeNode | None:
|
||
|
|
"""Build binary tree: Divide and conquer"""
|
||
|
|
# Terminate when subtree interval is empty
|
||
|
|
if r - l < 0:
|
||
|
|
return None
|
||
|
|
# Initialize root node
|
||
|
|
root = TreeNode(preorder[i])
|
||
|
|
# Query m to divide left and right subtrees
|
||
|
|
m = inorder_map[preorder[i]]
|
||
|
|
# Subproblem: build left subtree
|
||
|
|
root.left = dfs(preorder, inorder_map, i + 1, l, m - 1)
|
||
|
|
# Subproblem: build right subtree
|
||
|
|
root.right = dfs(preorder, inorder_map, i + 1 + m - l, m + 1, r)
|
||
|
|
# Return root node
|
||
|
|
return root
|
||
|
|
|
||
|
|
|
||
|
|
def build_tree(preorder: list[int], inorder: list[int]) -> TreeNode | None:
|
||
|
|
"""Build binary tree"""
|
||
|
|
# Initialize hash table, storing in-order elements to indices mapping
|
||
|
|
inorder_map = {val: i for i, val in enumerate(inorder)}
|
||
|
|
root = dfs(preorder, inorder_map, 0, 0, len(inorder) - 1)
|
||
|
|
return root
|
||
|
|
|
||
|
|
|
||
|
|
"""Driver Code"""
|
||
|
|
if __name__ == "__main__":
|
||
|
|
preorder = [3, 9, 2, 1, 7]
|
||
|
|
inorder = [9, 3, 1, 2, 7]
|
||
|
|
print(f"Pre-order traversal = {preorder}")
|
||
|
|
print(f"In-order traversal = {inorder}")
|
||
|
|
|
||
|
|
root = build_tree(preorder, inorder)
|
||
|
|
print("The built binary tree is:")
|
||
|
|
print_tree(root)
|