How to Fix Level Order Binary Tree Traversal Errors

In this tutorial, you'll learn about How to Fix Level Order Binary Tree Traversal Errors. We cover key concepts, practical examples, and best practices.

Fix level order binary tree traversal errors when BFS queue doesn't track levels separately.

Quick Fix

Wrong

def level(r):
    if not r: return []
    q,res=[r],[]
    while q:
        cur=q.pop(0); res.append(cur.val)
        if cur.left: q.append(cur.left)
        if cur.right: q.append(cur.right)
    return res

Flat traversal, not level-separated.

Right

from collections import deque
def level(r):
    if not r: return []
    q=deque([r]); res=[]
    while q:
        lvl=[]
        for _ in range(len(q)):
            cur=q.popleft(); lvl.append(cur.val)
            if cur.left: q.append(cur.left)
            if cur.right: q.append(cur.right)
        res.append(lvl)
    return res

[3,9,20,null,null,15,7] -> [[3],[9,20],[15,7]].

Prevention

Process all current queue nodes before next level. Track len(q) before iterating.

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