Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 6 | Brain Teasers
6.1 Add arithmetic operators (plus, minus, times, divide) to make the following expression
true: 3 1 3 6 = 8. You can use any parentheses you’d like.
SOLUTION
pg 60
An interviewer is asking this problem to see how you think and approach problems—so
don’t just guess randomly.
Try approaching this the following way: What sorts of operations would get us to 8? I can
think of a few:
4 * 2 = 8
16 / 2 = 8
4 + 4 = 8
Let’s start with the first one. Is there any way to make 3 1 3 6 produce 4 * 2? We can easily
notice that 3 + 1 = 4 (the first two numbers). We can also notice that 6 / 3 = 2. If we had “3 1
6 3”, we’d be done, since (3 + 1)*(6 / 3) = 8. Although it seems a little unconventional to do
this, we can, in fact, just flip the 6 and the 3 to get the solution:
(( 3 + 1 ) / 3) * 6 = 8
1 4 3
Cracking the Coding Interview | Concepts and Algorithms