← Snowflake Interview Insights
My first instinct was to do a preorder traversal but then I realized I needed the subtree sums, which means you actually want to compute bottom-up, so postorder made more sense.
Use a post-order traversal of T1 to compute subtree sums, storing each sum in the corresponding node of T2. Since the trees are structurally identical, you can traverse both simultaneously, achieving O(n) time and O(h) space (where h is tree height) with recursion.
Pro tip: Mention that you can avoid extra space by reusing T2's nodes to store the sums, and that an iterative post-order traversal can achieve O(1) extra space if you use parent pointers or Morris traversal.
Confirm that T1 and T2 are complete binary trees with identical structure, and that you need to modify T2 in-place so each node's value becomes the sum of the corresponding subtree in T1.
Select a post-order traversal (left, right, root) because subtree sums require children's sums before the parent. Traverse T1 and T2 simultaneously to map nodes.
Write a recursive function that takes corresponding nodes from T1 and T2. Recursively compute left and right subtree sums, then set T2 node's value to T1 node's value plus left and right sums.
State that time complexity is O(n) since each node is visited once. Space complexity is O(h) for recursion stack, where h is tree height; for a complete binary tree, h = O(log n).
Mention that an iterative post-order traversal can reduce space to O(1) if parent pointers exist, or O(h) otherwise. Also note that modifying T2 in-place avoids extra space for the result.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Pretty standard BFS construction, use a queue and assign left/right children as you pop nodes.
Clarify the problem constraints and edge cases, then explain that a complete binary tree can be built by mapping array indices to tree nodes using the relationship: for index i, left child is at 2i+1 and right child at 2i+2. Implement the construction iteratively using a queue or recursively, ensuring all nodes are linked correctly.
Pro tip: Mention that this index mapping works because a complete binary tree fills levels left-to-right, so the array representation is compact with no gaps. Also, discuss handling of null or empty input and potential integer overflow for large indices.
Restate the problem: given a level-order array, construct a complete binary tree. Clarify that the array represents a complete tree, so no null markers are needed except possibly for empty input.
Decide between iterative (using a queue) or recursive (using index mapping) construction. Explain the trade-offs: iterative is intuitive, recursive is concise but may risk stack overflow for deep trees.
For iterative: create root from first element, use a queue to track parent nodes, and assign left and right children from subsequent array elements. For recursive: define a helper that takes an index, creates a node if index is within bounds, and recursively builds left and right subtrees.
Walk through a small example (e.g., [1,2,3,4,5]) to verify the tree structure. Check edge cases: empty array, single element, and array with only left children.
State that both approaches run in O(n) time and O(n) space (for the tree and queue/recursion stack). Mention that the recursive approach uses O(h) stack space, where h is the height.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.