How to convert bottom-up recursive algorithm to iterative stack in JavaScript

Answer 1

What about something like this:

console.log(JSON.stringify(create(4), null, 2))

function create(depth) {
    let n = Math.pow(2, depth);
    let nodes = [];
    for (let i = 0; i < n; i++)
        nodes.push(new Klass(depth));
    for (depth--; depth >= 0; depth--) {
        let next = [];
        while (nodes.length > 0)
            next.push(new Klass(depth, nodes.pop(), nodes.pop()));
        nodes = next;
    }
    return nodes[0];
}

function Klass(i, l, r) {
  this.i = i
  this.l = l
  this.r = r
}

The call to get the same result would be create(4);. It is not exactly the same creation order, it creates the nodes from bottom to top, while recursive is like:

   7
 3   6
1 2 4 5

You can also mimick this behavior with a stack:

console.log(JSON.stringify(create(4), null, 2))

function create(depth) {
  let stack = [{depth: 0}]
  for (;;) {
    let i = stack.length - 1
    let cur = stack[i]
    if (typeof cur.left === 'undefined') {
      if (cur.depth < depth) {
        stack.push({depth: cur.depth + 1, parent: i, pos: 'right'})
        stack.push({depth: cur.depth + 1, parent: i, pos: 'left'})
      } else {
        stack[cur.parent][cur.pos] = new Klass(cur.depth)
        stack.pop()
      }
    } else {
      let node = new Klass(cur.depth, cur.left, cur.right)
      if (cur.depth == 0)
        return node
      stack[cur.parent][cur.pos] = node
      stack.pop()
    }
  }
}

function Klass(i, l, r) {
  this.i = i
  this.l = l
  this.r = r
}

The right node is pushed first on the stack and then the left node, so that the left node is higher in the stack and processed first.