def reconstruct_tree(preorder, inorder):
# TODO - you fill in here.
# 1. root is first node in preorder traversal
# 2. use root to divide inorder traversal into left and right subtrees (inorder)
# 3. use (left,right) inorder traversal set to partition preorder traversal
# after root element into (left,right) preorder traversal
#
print('preorder={},inorder={}'.format(preorder, inorder))
if not preorder:
print("returning None")
return None
# 1.
root_value = preorder[0]
root = BstNode(root_value)
# 2.
root_index = inorder.index(root_value)
# root_index = element_index[root_value]
print('root_index=', root_index)
# 3.
inorder_left_tree = inorder[:root_index]
inorder_right_tree = inorder[root_index + 1:]
print('inorder_left_tree={},inorder_right_tree={}'.format(
inorder_left_tree, inorder_right_tree))
preorder_left_tree = preorder[1:root_index + 1]
preorder_right_tree = preorder[root_index + 1:]
print('preorder_left_tree={},preorder_right_tree={}'.format(
preorder_left_tree, preorder_right_tree))
#
root.left = reconstruct_tree(preorder_left_tree, inorder_left_tree)
root.right = reconstruct_tree(preorder_right_tree, inorder_right_tree)
print('root={}'.format(root))
return root
def helper(start, end):
if end - start < 0:
return None
mid_idx = (end+start)//2
node = BstNode(A[mid_idx])
node.left = helper(start, mid_idx-1)
node.right = helper(mid_idx+1, end)
return node
def bst_helper(A, start, end):
if end < start:
return None
middle = (start + end) // 2
node = BstNode(A[middle])
node.left = bst_helper(A, start, middle - 1)
node.right = bst_helper(A, middle + 1, end)
return node
def build_min_height_bst_from_sorted_array(A: List[int]) -> Optional[BstNode]:
root = None
if len(A) != 0:
middle = len(A) // 2
root = BstNode(A[middle])
left_arr = A[:middle]
right_arr = A[middle + 1:]
root.left = build_min_height_bst_from_sorted_array(left_arr)
root.right = build_min_height_bst_from_sorted_array(right_arr)
return root
def rebuild_bst_from_preorder(preorder_sequence):
if len(preorder_sequence) == 0:
return None
root = BstNode(preorder_sequence[0])
i = 1
left = [x for x in preorder_sequence[1:] if x < preorder_sequence[0]]
right = [x for x in preorder_sequence[1:] if x > preorder_sequence[0]]
root.left = rebuild_bst_from_preorder(left)
root.right = rebuild_bst_from_preorder(right)
return root
def build_min_height_bst_from_sorted_subarray(start, end):
if start > end:
return None
mid = (start + end) // 2
tree = BstNode(A[mid])
tree.left = build_min_height_bst_from_sorted_subarray(start, mid - 1)
tree.right = build_min_height_bst_from_sorted_subarray(mid + 1, end)
return tree
def build_tree(A):
if not A:
return None
mid_i = (len(A) - 1) // 2
mid_elem = A[mid_i]
node = BstNode(mid_elem)
node.left = build_tree(A[:mid_i])
node.right = build_tree(A[mid_i + 1:])
return node
def helper(left_limit, right_limit):
nonlocal idx
nonlocal arr
if idx >= len(arr) or not left_limit < arr[idx] < right_limit:
return None
node = BstNode(data=arr[idx])
idx += 1
node.left = helper(left_limit, node.data)
node.right = helper(node.data, right_limit)
return node
def preorder_helper(lower_bound, upper_bound):
if root_idx[0] == len(preorder_sequence):
return None
root = preorder_sequence[root_idx[0]]
if not lower_bound <= root <= upper_bound:
return None
root_idx[0] += 1
node = BstNode(root)
## ordering is critical
node.left = preorder_helper(lower_bound, root)
node.right = preorder_helper(root, upper_bound)
return node
def rebuild_within_range(low=float('-inf'), high=float('inf')):
nonlocal i
if i >= len(preorder_sequence):
return None
v = preorder_sequence[i]
if low <= v <= high:
u = BstNode(v)
i += 1
u.left = rebuild_within_range(low=low, high=v)
u.right = rebuild_within_range(low=v, high=high)
return u
return None
def build_min_height_bst_from_sorted_array(A):
# TODO - you fill in here.
# Alok
if not A:
return None
splitIndex = len(A)//2
root = BstNode(A[splitIndex])
root.left = build_min_height_bst_from_sorted_array(A[:splitIndex])
root.right = build_min_height_bst_from_sorted_array(A[splitIndex+1:])
return root
def construct_tree(root_idx, lower_bound, upper_bound):
if root_idx == len(preorder_sequence):
return Result(None, root_idx)
root_val = preorder_sequence[root_idx]
if not lower_bound <= root_val <= upper_bound:
return Result(None, root_idx)
root_idx += 1
tree = BstNode(root_val)
tree.left, root_idx = construct_tree(root_idx, lower_bound, root_val)
tree.right, root_idx = construct_tree(root_idx, root_val, upper_bound)
return Result(tree, root_idx)
def build_bst(seq, start, end):
if start == end:
return None
node = BstNode(seq[start])
right = start
while right < end and seq[right] <= seq[start]:
right += 1
node.left = build_bst(seq, start + 1, right)
node.right = build_bst(seq, right, end)
return node
def build_tree(start, end):
if start > end:
return None
root_val = preorder_sequence[start]
right_index = start + 1
while right_index <= end:
if preorder_sequence[right_index] > root_val:
break
right_index += 1
tree = BstNode(root_val)
tree.left = build_tree(start + 1, right_index - 1)
tree.right = build_tree(right_index, end)
return tree
def rebuild_bst_from_preorder(
preorder_sequence: List[int]) -> Optional[BstNode]:
if len(preorder_sequence) == 0: return None
root = BstNode(preorder_sequence[0])
searchPath = deque([root], maxlen=len(preorder_sequence))
i = 1
while i < len(preorder_sequence):
if preorder_sequence[i] < preorder_sequence[i - 1]:
node = BstNode(preorder_sequence[i])
searchPath[-1].left = node
searchPath.append(node)
else:
node = searchPath.pop()
while len(
searchPath) and searchPath[-1].data < preorder_sequence[i]:
node = searchPath.pop()
right = BstNode(preorder_sequence[i])
node.right = right
searchPath.append(right)
i += 1
return root
def binary_tree_from_preorder_inorder(preorder, inorder):
# for a preorder traversal, the first element is the root,
# followed by a set of elements for the left subtree then the right
# the set of left subtree elements in the preorder traversal can be determined by
# using the root element to divide the inorder traversal elements into
# left and right subtree elements,
# the left subtree elements in the inorder traversal is the elements that
# preceeds the root element in the inorder traversal
if not preorder:
return None
root_val = preorder[0]
root = BstNode(root_val)
inorder_root_index = inorder.index(root_val)
inorder_left_tree = inorder[:inorder_root_index]
inorder_right_tree = inorder[inorder_root_index + 1:]
preorder_left_tree = preorder[1:(len(inorder_left_tree) + 1)] #skip root
preorder_right_tree = preorder[(len(inorder_left_tree) + 1):]
root.left = binary_tree_from_preorder_inorder(preorder_left_tree,
inorder_left_tree)
root.right = binary_tree_from_preorder_inorder(preorder_right_tree,
inorder_right_tree)
return root