Blind 75 / Trees / Binary Tree Level Order Traversal

Binary Tree Level Order Traversal

MediumTrees
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Problem

Given the root of a binary tree, return the values of its nodes grouped by level, from left to right, as a list of lists.

Examples

Example 1
Input:root = [3,9,20,null,null,15,7]
Output:[[3],[9,20],[15,7]]
Example 2
Input:root = [1]
Output:[[1]]

Constraints

  • The number of nodes is in the range [0, 2000].

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How to approach it: the Trees pattern

A tree is a hierarchical structure where each node has at most a fixed number of children (two, for binary trees). Almost every tree problem reduces to a traversal — decide what order you visit nodes in, and what you do at each one.

Look for this pattern when

  • The problem is naturally recursive: "the answer for this tree depends on the answer for its subtrees."
  • You need level-by-level information (BFS) versus depth-first structural information (DFS).

Read the full Trees guide →

Video walkthroughs

Original problem on LeetCode ↗

More Trees problems

← Lowest Common Ancestor of a Binary Search TreeValidate Binary Search Tree →