Return last item first in a recursive function

Question

I need a recursive function that returns the deepest item first because reversing array and pushing to first position are slow.

I have an object:

const myObject = {
    id: 3,
    parent: {
        id: 2,
        parent: {
            id: 1,
            parent: null,
        },
    },
};

And a recursive function:

function findParents(myObject, parents = []) {
  if (myObject.parent) {
    parents.push(myObject.parent.id);
    return findParents(myObject.parent, parents);
  }

  return parents; // [2, 1]
}

I need a recursive function that returns me an array of an objects parent id's so that the last parent is first in the returned array. So for the above example, if I pass in that object into my function, it should return the id's of the parents like this:

[1, 2]

Well if you have the algorithm for pushing onto the array the normal order, you can just use `array.unshift` instead of `array.push`. Unshift pushes them onto the front. — TKoL, Oct 30 '17 at 17:34
In other words, just change this `parents.push(myObject.parent.id);` to `parents.unshift(myObject.parent.id);` — TKoL, Oct 30 '17 at 17:35
Yes but unshift is 10x slower: https://stackoverflow.com/a/26370620/3743266 — user3743266, Oct 30 '17 at 17:35
But how big is your list? It might be irrelevant. 10x slower when it takes 0.01 second instead of 0.001s is imperceptible. — msanford, Oct 30 '17 at 17:36
@user3743266 when i first posted my answer, it ws wrong. Ive changed it now — TKoL, Oct 30 '17 at 17:41

score 2 · Accepted Answer · answered Oct 30 '17 at 17:37

2

function findParents(myObject, parents = []) {
  if (myObject.parent) {
    parents = findParents(myObject.parent, parents);
  }
  parents.push(myObject.id)
  return parents; // [2, 1]
}

answered Oct 30 '17 at 17:37

TKoL

13,158
3
39
73

this returns [1] – user3743266 Oct 30 '17 at 17:41
Thanks, so it was a simple logic error, i couldnt figure it out. Also why do people randomly downvote without any reason? I thought my question was pretty clear – user3743266 Oct 30 '17 at 17:47

Return last item first in a recursive function

1 Answers1