LeetCode 958: Check Completeness of a Binary Tree( BFS ) | lecture 14 | Tree playlist | DSA

LeetCode 958: Check Completeness of a Binary Tree (The BFS Queue Trick!) ​Today we are tackling LeetCode 958: Check Completeness of a Binary Tree, a popular medium-level problem that tests your ability to strictly enforce structural rules in a tree. ​A binary tree is complete if every level except possibly the last is fully filled, and all nodes in the last level are as far left as possible. While this sounds tricky to check recursively, it becomes incredibly simple when you use Breadth-First Search (BFS). In this video, we will look at how a standard queue can help us spot structural gaps instantly! ​📌 What We Will Cover: ​🔍 What Makes a Tree Complete? A quick visual refresher on the "no gaps allowed" rule. ​🥞 The Level-Order Insight: Why processing the tree level-by-level (BFS) is the perfect strategy for this problem. ​🚫 The Null Marker Strategy: How allowing null values into our BFS queue acts as a tripwire to catch invalid gaps. ​🏁 The "No Turning Back" Rule: The exact condition that tells us our tree is incomplete (seeing a valid node after we’ve already encountered a null inside our BFS traversal). ​💻 Step-by-Step Code Walkthrough: Writing clean, readable code and analyzing why it runs in O(N) Time. ​💬 Challenge Question: ​Why does the BFS approach make it so easy to stop early as soon as we hit an empty spot? Could you think of a scenario where BFS might use too much memory? Let me know your thoughts in the comments below! ​If this video made checking tree completeness using BFS crystal clear, make sure to hit that Like button and Subscribe for more simplified interview prep! ​#LeetCode #BinaryTree #BreadthFirstSearch #BFS #DataStructures #Algorithms #CodingInterview #FAANG #LearnToCode