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December 4, 2021

In software, we rarely have meaningful requirements. Even if we do, the only measure of success that matters is whether our solution solves the customer’s shifting idea of what their problem is.

- Jeff Atwood

Day 9: Lowest Common Ancestor

In the official React documentation, it's recommended that to find out where state data should live, the following steps be taken:

  • Identify every component that renders something based on that state.
  • Find a common owner component (a single component above all the components that need the state in the hierarchy).
  • Either the common owner or another component higher up in the hierarchy should own the state.

This is an algorithmic problem-- let's find the lowest common ancestor!

You're given a binary search tree and two of its child nodes as parameters. Can you write a method lowestCommonAncestor(root: Node, node1: Node, node2: Node) to identify the lowest common ancestor of the two nodes? The lowest common ancestor is the deepest node that has both of the two nodes as descendants.

In the below example, the lowest common ancestor of node 5 and 8 is 7.

      / \
     4   8
    / \
   1   5

The method will called in the following manner: lowestCommonAncestor(root, node1, node2);.

You can assume our standard node definition for the tree vertices:

function Node(val) {
  this.val = val;
  this.left = this.right = null;


  • Number of nodes in the BST <= 1000
  • The nodes will always contain integer values between -1000000000 and 1000000000
  • Expected time complexity : O(n)
  • Expected space complexity : O(1)

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Hands Down the Best Systems Design Resource?

I was perusing Hacker News last week when I encountered a link subtlely titled "System Design Interview Book Review". Curious, I checked out the article-- it was a review of a resource I'd never heard of called, "System Design Interview: an Insider's Guide".

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I checked the course out and found it to be the perfect compliment to AlgoDaily. The same way we focus our learning around problems, Systems Design Interview focuses on real world use cases. What I love is the systems covered include ones you'll likely build as a software engineer in 2021: a web crawler, news feed, autocomplete, Google Drive, key-value store, etc.

Check out "System Design Interview: an Insider's Guide" today! I highly recommend it.

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