#52 Medium Time O(n) Space O(n) Animated

Lowest Common Ancestor of a BST

Find the LCA of two nodes in a BST.

LeetCode ↗

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1 Predict
2 Trace
3 Explain
4 Implement

Pattern Tree traversal with recursive contracts

Ask each subtree for exactly the information its parent needs, then combine the two answers.

Approach: BST Properties (Iterative)

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Core Insight

Ask each subtree for exactly the information its parent needs, then combine the two answers.

Common Pitfall

Define the base case before combining left and right subtree results.

Description

Find the LCA of two nodes in a BST.

Constraints

Time Complexity: O(n)
Space Complexity: O(n)

Solution — BST Properties (Iterative)

TimeO(n) SpaceO(n)

Implementation Plan

Translate the invariant into these small, testable moves.

Define what one recursive call returns.
Handle an empty node.
Recurse into children.
Combine the child results at the current node.

def lowestCommonAncestor(root, p, q):
    curr = root
    
    while curr:
        if p.val > curr.val and q.val > curr.val:
            curr = curr.right
        elif p.val < curr.val and q.val < curr.val:
            curr = curr.left
        else:
            return curr

Subtree of Another Tree

Binary Tree Level Order Traversal

Lowest Common Ancestor of a BST

Steps

Description

Constraints

Tags

Solution — BST Properties (Iterative)