# Blog Archives

## Getting a fair result with an unfair coin

How can you get a fair coin toss if someone hands you a coin that is weighted to come up heads more often than tails?

We are NY Tech asks: “How many unique areas of human knowledge have the right size of passionate users to make it as a Stack Exchange site?” Answer: 30,000.

## Storing 1 million phone numbers

What is the most efficient way, memory-wise, to store 1 million phone numbers? Apparently this is an interview question at Google, although this seems like its a bit too easy.

## Reverse a String

A typical programming interview question is “reverse a string, in place”. if you understand pointers, the solution is simple. even if you don’t, it can be accomplished using array indices. i usually ask candidates this question first, so they get the algorithm in their head. then i play dirty by asking them to reverse the … Read More

## 100 Doors in a Row

Problem: you have 100 doors in a row that are all initially closed. you make 100 passes by the doors starting with the first door every time. the first time through you visit every door and toggle the door (if the door is closed, you open it, if its open, you close it). the second … Read More

## Red Marbles, Blue Marbles

Problem: you have two jars, 50 red marbles, 50 blue marbles. you need to place all the marbles into the jars such that when you blindly pick one marble out of one jar, you maximize the chances that it will be red. (when picking, you’ll first randomly pick a jar, and then randomly pick a … Read More

## Bumblebee

problem: two trains enter a tunnel 200 miles long (yeah, its a big tunnel) travelling at 100 mph at the same time from opposite directions. as soon as they enter the tunnel a supersonic bee flying at 1000 mph starts from one train and heads toward the other one. as soon as it reaches the … Read More

## int atoi( char* pStr )

Problem: write the definition for this function without using any built-in functions. if pStr is null, return 0. if pStr contains non-numeric characters, either return 0 (ok) or return the number derived so far (better) (e.g. if its “123A”, then return 123). assume all numbers are positive. plus or minus signs can be considered non-numeric … Read More

## Daughters’ Ages

Two MIT math grads bump into each other at Fairway on the upper west side. They haven’t seen each other in over 20 years. the first grad says to the second: “how have you been?”second: “great! i got married and i have three daughters now”first: “really? how old are they?”second: “well, the product of their … Read More

## Palindromes

Problem: this year on October 2, 2001, the date in MMDDYYYY format will be a palindrome (same forwards as backwards).10/02/2001when was the last date that this occurred on? (see if you can do it in your head!) Solution olution: we know the year has to be less than 2001 since we already have the palindrome … Read More