N-ary Tree Level Order Traversal - Video Solutions

Problem Overview: Given the root of an N-ary tree, return the level order traversal of its nodes' values. Nodes at the same depth should appear in the same list, processed from left to right.

Approach 1: Iterative Level Order Traversal using Queue (O(n) time, O(w) space)

This approach uses Breadth-First Search (BFS) with a queue to process the tree level by level. Start by pushing the root into the queue. For each level, record the current queue size, iterate that many nodes, and enqueue all children of each node. This guarantees that nodes are grouped by depth before moving to the next level. Time complexity is O(n) because every node is visited exactly once, and space complexity is O(w), where w is the maximum width of the tree due to the queue storing nodes of the current level.

This pattern appears frequently in Breadth-First Search problems and works naturally for level-based traversal. If you already think in BFS layers, this solution is straightforward and easy to implement.

Approach 2: Recursive Level Order Traversal (O(n) time, O(h) space)

The recursive approach uses Depth-First Search (DFS) while tracking the current depth. Maintain a result list where index i stores values for level i. During recursion, if the current depth does not yet exist in the result, create a new list. Append the node's value and recursively process each child with depth + 1. Each node is still visited once, giving O(n) time complexity. Space complexity becomes O(h) for the recursion stack, where h is the tree height.

This approach blends tree traversal with recursion. It avoids an explicit queue and keeps the implementation compact, though it may be less intuitive if you naturally think about levels iteratively.

Recommended for interviews: The queue-based BFS solution is the expected answer in most interviews because level order traversal directly maps to BFS mechanics. The recursive variant shows deeper understanding of traversal patterns, but interviewers typically prefer the iterative BFS since it clearly expresses level boundaries.