Python树的重建实现示例

class TreeNode:
    def __init__(self, value):
        self.val = value
        self.left = None
        self.right = None

def build_tree(preorder, inorder):
    if not preorder or not inorder:
        return None
    
    root_val = preorder[0]
    root = TreeNode(root_val)
    
    index = inorder.index(root_val)
    
    root.left = build_tree(preorder[1:index + 1], inorder[:index])
    root.right = build_tree(preorder[index + 1:], inorder[index + 1:])
    
    return root

from collections import deque

def build_tree_level_order(level_order):
    if not level_order:
        return None
    
    root = TreeNode(level_order[0])
    queue = deque([root])
    i = 1
    
    while i < len(level_order):
        current = queue.popleft()
        
        left_val = level_order[i]
        i += 1
        if left_val is not None:
            current.left = TreeNode(left_val)
            queue.append(current.left)
        
        right_val = level_order[i]
        i += 1
        if right_val is not None:
            current.right = TreeNode(right_val)
            queue.append(current.right)
    
    return root

preorder = [3, 9, 20, 15, 7]
inorder = [9, 3, 15, 20, 7]

root = build_tree(preorder, inorder)

# 验证重建的树
def inorder_traversal(root):
    if root is not None:
        inorder_traversal(root.left)
        print(root.val, end=" ")
        inorder_traversal(root.right)

print("Inorder Traversal of Reconstructed Tree:")
inorder_traversal(root)

level_order = [3, 9, 20, None, None, 15, 7]

root_level_order = build_tree_level_order(level_order)

# 验证重建的树
def inorder_traversal_level_order(root):
    if root is not None:
        inorder_traversal_level_order(root.left)
        print(root.val, end=" ")
        inorder_traversal_level_order(root.right)

print("Inorder Traversal of Reconstructed Tree from Level Order:")
inorder_traversal_level_order(root_level_order)