Check Completeness of a Binary Tree

To determine if a binary tree is complete, I would use a Breadth-First Search approach because it processes nodes level by level, which aligns perfectly with the definition of completeness. First, I'd initialize a queue and add the root. Then, I iterate through the queue. For each node, I check its children. If I encounter a null child, I set a flag indicating that we have reached the end of the tree structure. Crucially, once this flag is set, any subsequent non-null node found in the queue indicates a gap, meaning the tree is not complete. For example, consider a tree where the root has a left child but no right child, yet the left child has a right grandchild. In this case, after processing the root's missing right child, we hit our 'null' condition. However, when we later process the left child's right grandchild, we find a non-null node after the flag was triggered, correctly identifying the incompleteness. This method runs in O(N) time since we visit every node once, and uses O(W) space for the queue, where W is the maximum width. This solution is robust, avoids recursion depth issues, and handles the specific structural constraints Adobe values in their engineering roles.