[Tree] Depth First Values

class Node {
  constructor(val) {
    this.val = val;
    this.left = null;
    this.right = null;
  }
}

const depthFirstValues = (root) => {
	// think of base case & return type
	if(root === null) return []

	return [root.val, ...depthFirstValues(root.left), ...depthFirstValues(root.right)]
};

const depthFirstValuesStack = (root) => {
	// special case handling
	if(root === null) return []

	const stack = [root]
	const res = []

	while(stack.length > 0) {
		const current = stack.pop()
		res.push(current.val)

		if(current.right) stack.push(current.right)
		if(current.left) stack.push(current.left)
	}

	return res
}

{
	const a = new Node('a');
	const b = new Node('b');
	const c = new Node('c');
	const d = new Node('d');
	const e = new Node('e');
	const f = new Node('f');

	a.left = b;
	a.right = c;
	b.left = d;
	b.right = e;
	c.right = f;

//      a
//    /   \
//   b     c
//  / \     \
// d   e     f

	console.log(
	depthFirstValues(a));
	console.log(
		depthFirstValuesStack(a));
//    -> ['a', 'b', 'd', 'e', 'c', 'f']
}

{
	const a = new Node('a');
	const b = new Node('b');
	const c = new Node('c');
	const d = new Node('d');
	const e = new Node('e');
	const f = new Node('f');
	const g = new Node('g');

	a.left = b;
	a.right = c;
	b.left = d;
	b.right = e;
	c.right = f;
	e.left = g;

//      a
//    /   \
//   b     c
//  / \     \
// d   e     f
//    /
//   g

	console.log(
	depthFirstValues(a));
	console.log(
		depthFirstValuesStack(a));
//    -> ['a', 'b', 'd', 'e', 'g', 'c', 'f']
}

{
	const a = new Node('a');
//      a
	console.log(
	depthFirstValues(a));
	console.log(
		depthFirstValuesStack(a));
//    -> ['a']
}

{
	const a = new Node('a');
	const b = new Node('b');
	const c = new Node('c');
	const d = new Node('d');
	const e = new Node('e');

	a.right = b;
	b.left = c;
	c.right = d;
	d.right = e;

//      a
//       \
//        b
//       /
//      c
//       \
//        d
//         \
//          e

	console.log(
	depthFirstValues(a));
	console.log(
		depthFirstValuesStack(a));
//    -> ['a', 'b', 'c', 'd', 'e']
}

{
	console.log(
	depthFirstValues(null));
	console.log(
		depthFirstValuesStack(null));
//    -> []
}