### Binary Tree Level Order Traversal #### Problem Statement Given the root node of a binary tree, perform a level-order traversal of the tree. This involves visiting each node at the current level before moving on to the next level. #### Rules and Constraints * The binary tree may be empty, in which case the function should return an empty result. * Nodes of the binary tree are referenced by pointers that point to the left child and right child of a node. * The function should process each node from the root node to the leaf nodes, visiting each node before moving on to its children. * For each level, the order of visitation is typically from left to right. * The level-order traversal algorithm may use additional data structures that store nodes in order of their levels. #### Time and Space Complexity * The time complexity should be O(N), where N is the number of nodes in the binary tree. This is because each node is visited exactly once. * The space complexity should be O(W), where W is the maximum width of the binary tree. This is because in the worst case, the function needs to store all nodes at the widest level in the result data structure. In a balanced binary tree, the space complexity would be O(log N), but this is a less common scenario. ### Example **Input:** {"root":[3,9,20,null,null,15,7]} **Output:** [[3],[9,20],[15,7]]