Check if a binary tree is complete

You are given the root of a binary tree. Determine whether the tree is a **complete binary tree**. A binary tree is complete if: - Every level, except possibly the last, is completely filled. - All nodes in the last level are as far left as possible. ### Input - `root`: root node of a binary tree (or `null`). ### Output - Return `true` if the tree is complete; otherwise return `false`. ### Constraints - Number of nodes: `0..10^5` - Node values are irrelevant to completeness. ### Notes - Aim for an algorithm that is correct for large trees; discuss time complexity and space usage (e.g., typical BFS approach vs. any space optimizations).