How to Fix Lowest Common Ancestor in BST Errors

In this tutorial, you'll learn about How to Fix Lowest Common Ancestor in BST Errors. We cover key concepts, practical examples, and best practices.

Fix lowest common ancestor in bst errors when ancestor search doesn't use BST properties or direction inverted.

Quick Fix

Wrong

def lca(r,p,q):
    if not r: return None
    if r.val>p.val and r.val>q.val: return lca(r.right,p,q)

Wrong direction: both values less -> should go LEFT not right.

Right

def lca(r,p,q):
    cur=r
    while cur:
        if p.val<cur.val and q.val<cur.val: cur=cur.left
        elif p.val>cur.val and q.val>cur.val: cur=cur.right
        else: return cur
    return None

BST [6,2,8,0,4,7,9,null,null,3,5], p=2,q=8 -> 6. O(log n).

Prevention

If both values < root, go left. Both > root, go right. Otherwise root is LCA.

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