#53 Medium Time O(n) Space O(n) Animated

Binary Tree Level Order Traversal

Return level-order traversal of binary tree's node values.

LeetCode ↗

Tutor Mode Build the mental model before you memorize the code.

1 Predict
2 Trace
3 Explain
4 Implement

Pattern Tree traversal with recursive contracts

Ask each subtree for exactly the information its parent needs, then combine the two answers.

Approach: BFS (Queue)

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Core Insight

Ask each subtree for exactly the information its parent needs, then combine the two answers.

Common Pitfall

Define the base case before combining left and right subtree results.

Description

Return level-order traversal of binary tree's node values.

Constraints

Time Complexity: O(n)
Space Complexity: O(n)

Solution — BFS (Queue)

TimeO(n) SpaceO(n)

Implementation Plan

Translate the invariant into these small, testable moves.

Define what one recursive call returns.
Handle an empty node.
Recurse into children.
Combine the child results at the current node.

def levelOrder(root):
    res = []
    q = collections.deque()
    if root:
        q.append(root)
        
    while q:
        level = []
        for i in range(len(q)):
            node = q.popleft()
            level.append(node.val)
            if node.left:
                q.append(node.left)
            if node.right:
                q.append(node.right)
        res.append(level)
        
    return res

Lowest Common Ancestor of a BST

Binary Tree Right Side View

Binary Tree Level Order Traversal

Steps

Description

Constraints

Tags

Solution — BFS (Queue)