Code Corner: Triangles

Code Corner: Triangles

For the last two weeks, I’ve had a bunch of interviews and code challenge tasks I was taking which took up a lot of my time. Add that to #DadLife and the result is two weeks with no content. The pressure is off for the moment so I’m picking it up again. So for today:

Problem

We are given an N length array with numbers ranging between a few negative million to a few positive million. If there exists at least one 3-tuple (P, Q, R) combination of values that satisfy the conditions below, return 1 otherwise return 0.

Conditions

  • The values A, B, C represent indices of A
  • 0 <= P < Q < R < N
  • A[P] + A[Q] > A[R]
  • A[P] + A[R] > A[Q]
  • A[R] + A[Q] > A[P]

My Solution

Brute forcing…

The brute force approach worked ok, but as usual didn’t make the performance tests.

Here we have to check a few conditions.

  • First and easiest, A[P] should always be less than the sum of A[Q] and A[R]. a < b + c
  • Second A[P] should be greater than A[Q] – A[R] and A[P] should be greater than A[R] – A[Q], we get these from solving the equations we get in the conditions and can combine the two into one check a > Math.abs(b - c)

And thus our first attempt looks like:

function brute_force(A) { const isTriangle = (a, b, c) => { return a > Math.abs(b - c) && a < b + c } if (A.length < 3) { return 0 } for (let i = 0; i < A.length - 2; i++) { for (let j = i + 1; j < A.length - 1; j++) { for (let k = j + 1; k < A.length; k++) { if (isTriangle(A[i], A[j], A[k])) { return 1 } } } } return 0 }
Code language: JavaScript (javascript)

Improved Solution

We are obviously looping too much. So I decided to optimise this by first introducing a sort. This should then hopefully reduce it down to one loop. Here is where I made a terrible mistake that messed me up for the better part of 2 hours.

Do you see that condition that checks if our three numbers match our conditions? Yeah, when switching to a sorted approach I was using I as my iterating variable and accidentally inserted 1 instead of i on one of my conditions. So instead of this:

if (A[i] > Math.abs(A[i + 1] - A[i + 2]) && A[i] < A[i + 1] + A[i + 2]) { return 1 }
Code language: JavaScript (javascript)

I had this:

if (A[i] > Math.abs(A[i + 1] - A[1 + 2]) && A[i] < A[i + 1] + A[1 + 2]) { return 1 }
Code language: JavaScript (javascript)

If you can’t see the difference, welcome to the club. So it passed a few local tests by happy accident but wouldn’t pass the larger tests. I spent a lot of time trying to figure out what was wrong until I was pulling my locks out. Then after a little break, I came back and saw it.

Correcting the typo fixed the issue and the code passed everything.

function solution(A) { if (A.length < 3) { return 0 } A.sort((a, b) => a - b) for (let i = 0; i < A.length - 2; i++) { if (A[i] > Math.abs(A[i + 1] - A[i + 2]) && A[i] < A[i + 1] + A[i + 2]) { return 1 } } return 0 }
Code language: JavaScript (javascript)

Typos are evil.


Leave a Reply

Your email address will not be published.