How to Fix Balanced Binary Tree Errors

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

Fix balanced binary tree errors when height difference calculation fails or early termination missed.

Quick Fix

Wrong

def bal(r):
    if not r: return True
    return bal(r.left) and bal(r.right)

Only checks subtrees but never verifies height at current node.

Right

def bal(r):
    def ch(n):
        if not n: return 0
        l=ch(n.left); r=ch(n.right)
        if l==-1 or r==-1 or abs(l-r)>1: return -1
        return 1+max(l,r)
    return ch(r)!=-1

[1,2,2,3,3,null,null,4,4] -> False. O(n).

Prevention

Return -1 sentinel for unbalanced. Early-terminate propagate -1 upward.

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