DSA · Theory

Lowest Common Ancestor of a Binary Tree

Theory

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Lowest Common Ancestor of a Binary Tree

Lowest Common Ancestor of a Binary Tree is a DSA idea you prove with folder on disk, not with a slogan.

Keep Lowest Common Ancestor of a Binary Tree small. Rohit should finish folder on disk in a few lines, not a 40-line dump.

Lowest Common Ancestor of a Binary Tree exists so Rohit can keep folder on disk correct in a file explorer.

Rohit ships Lowest Common Ancestor of a Binary Tree in a file explorer. That is the use case worth saying.

Don’t do this with Lowest Common Ancestor of a Binary Tree: unbalanced BST becoming a list. Interviewers spot it in ten seconds.

After Lowest Common Ancestor of a Binary Tree, Rohit should still remember unbalanced BST becoming a list.

One breath for Lowest Common Ancestor of a Binary Tree, then folder on disk, then unbalanced BST becoming a list. Sit down.

Exam tip

Board: folder on disk. Dry-run Lowest Common Ancestor of a Binary Tree. Name the trap: unbalanced BST becoming a list.

Example

class Node:
    def __init__(self, val, left=None, right=None):
        self.val, self.left, self.right = val, left, right

def inorder(n):
    if not n:
        return []
    return inorder(n.left) + [n.val] + inorder(n.right)

root = Node(2, Node(1), Node(3))
print(inorder(root))

Lowest Common Ancestor of a Binary Tree — inorder walks left → node → right (sorted order for a BST).

Short notes

DefLowest Common Ancestor of a Binary Tree — Rohit uses it for folder on disk in a file explorer.
RuleLowest Common Ancestor of a Binary Tree → left < root < right.
RememberLowest Common Ancestor of a Binary Tree + a dry-run table (a file explorer).
UseLowest Common Ancestor of a Binary Tree in a file explorer (folder on disk).
TrapLowest Common Ancestor of a Binary Tree — unbalanced BST becoming a list.
ExLowest Common Ancestor of a Binary Tree → folder on disk.

class Node: def __init__(self, val, left=None, right=None): self.val, self.left, self.right = val, left, right def inorder(n): if not n: return [] return inorder(n.left) + [n.val] + inorder(n.right) root = Node(2, Node(1), Node(3)) print(inorder(root))

Lowest Common Ancestor of a Binary Tree

Example

Short notes

Questions

Lowest Common Ancestor of a Binary Tree

Example

Short notes

Questions