PPractick

The problem

An inventory system models warehouses as a binary tree. Every warehouse's stock count must equal the sum of its two sub-warehouses' counts, with a missing child counting as 0, and leaves hold whatever they hold. An auditor must verify that every internal node satisfies this "children sum property". A traversal checks each node against its children. The whole tree passes only if every internal node does.

Example: [10, 8, 2, 3, 5, 1, 1] → true (10 = 8 + 2, 8 = 3 + 5, 2 = 1 + 1).

Function signature

bool isSumProperty(TreeNode* root);

Test cases

Input (level order)	Output	Explanation
[10, 8, 2, 3, 5, 1, 1]	true
[35, 20, 15, 15, 5, 10, 5]	true
[1, 4, 3, 5]	false	1 ≠ 4 + 3
[3, 1, 2, 2]	false	1 has a single child 2
[5]	true	A leaf has no constraint
[]	true

(Confirm: the rule applies to every node with at least one child; a missing child counts as 0; leaves are always fine.)

How interviewers phrase it

• "Check whether every node's value equals the sum of its children's values" (GeeksforGeeks "Children Sum in a Binary Tree").
• A harder variant: "increment node values (never decrement) so the tree satisfies the property".

Clarifying questions to ask

1. How do leaves behave? (They satisfy it automatically.)
2. Does a missing child count as 0? (Yes.)
3. Return a bool, or 1/0? (Either.)

Follow-ups to expect

1. "Recursive structure?"
2. "Iterative?"
3. "How would you convert any tree to satisfy it using only increments?"
4. "Could values overflow?"
5. "Complexity?"

Related questions

• Maximum Depth in BT Medium
• Check for balanced binary tree Medium
• Maximum path sum Medium
• Check if two trees are identical or not Medium
• Boundary Traversal Medium
• Vertical Order Traversal Medium

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