Pre-increment in recursive call

Question

I was solving this leetcode question https://leetcode.com/problems/binary-tree-right-side-view/description/ .

The following code works correctly.

class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List <Integer> ans = new LinkedList<>();
        if (root == null) return ans;
        traverse(root, ans, 0);
        return ans;
    }

    public void traverse(TreeNode root, List<Integer> ans, int currDepth){
        if (root == null) return;

        if (ans.size() == currDepth) ans.add(root.val);
        traverse(root.right, ans, currDepth + 1);
        traverse(root.left, ans, currDepth + 1);
    }
}

However, during the last 2 recursive calls, if I change the lines to

 traverse(root.right, ans, ++currDepth);
 traverse(root.left, ans, ++currDepth);

the code fails, why does this happen? Shouldn't both versions be equivalent?

increase the given value of currDepth by one before the recursive call finishes? As opposed to currDepth++ which would increase it afterwards — George Francis, Sep 27 '18 at 01:32
Great. Now what's the value of `currDepth` when used as an argument in the second invocation of `traverse` if the first increased it by one? — Sotirios Delimanolis, Sep 27 '18 at 01:33
Well, 2, right? It started as 0, first call increased it to 1, so during the 2nd call it should be 2 — George Francis, Sep 27 '18 at 01:35
Right. Did the you expect the same thing to happen when you used `currDept + 1`? — Sotirios Delimanolis, Sep 27 '18 at 01:36
You expected `currDept + 1` to increase the value stored in `currDept`? Then that's where your misunderstanding is. The `+` operator does not modify its operands. — Sotirios Delimanolis, Sep 27 '18 at 01:39
I mean, I know it wouldn't increase it during the current call, but rather on the next one, the one being called. Do you mean that using currDepth + 1 with an initial value of currDepth = 0 not make the value of currDepth = 1 in the next call? — George Francis, Sep 27 '18 at 01:45
Yes, I mean that two consecutive `currDepth + 1` expressions will result in the same value, whereas two consecutive `currDepth++` expressions will result in two different values. — Sotirios Delimanolis, Sep 27 '18 at 01:46
Ahh, now I see the error, this seems kinda dumb now haha, thanks. — George Francis, Sep 27 '18 at 01:48
calling "currdept + 1" will not change the values of currdepth (which is what you want); calling ++currDepth will change the values of currDepth (which is what you do not want). — Angel Koh, Sep 27 '18 at 01:48

score 1 · Accepted Answer · answered Sep 27 '18 at 01:45

Let us say currDepth = 0

In your first version, the two recursive calls will look like this:

traverse(root.right, ans, 1);
traverse(root.left, ans, 1);

This is correct because you want both recursive calls to go to the next level.

In your second version, it looks like this:

traverse(root.right, ans, 1);
traverse(root.left, ans, 2);

This means the first recursive call works fine, but the second one is wrong (skips one level).

Why? you changed your currDepth parameter. The first version of your code doesn't change it. It passes currDepth + 1 to the next level.

Thank you, I now see what went wrong, I forgot that using ++currDepth would change the value on the next line. — George Francis, Sep 27 '18 at 01:49

Pre-increment in recursive call

1 Answers1