[Algorithm] Binary tree: Level Order Traversal

function Node(val) {
  return {
    val,
    left: null,
    right: null
  };
}

function Tree() {
  return {
    root: null,
    addLeft(val, root) {
      const newNode = Node(val);
      root.left = newNode;
      return newNode;
    },
    addRight(val, root) {
      const newNode = Node(val);
      root.right = newNode;
      return newNode;
    },
    levelOrder(root, visitFn) {
      const helper = (node, visitFn) => {
        if (node === null) {
          return;
        }

        const q = new Queue();
        q.push(node);

        while (!q.isEmpty()) {
          const current = q.peak();
          visitFn(current.val);
          current.left && q.push(current.left);
          current.right && q.push(current.right);
          q.pop();
        }
      };

      helper(root, visitFn);
    }
  };
}

function Queue() {
  return {
    nodes: [],
    push(val) {
      this.nodes.push(val); // O(1)
    },
    pop() {
      const [first, ...rest] = this.nodes;
      this.nodes = rest;
      return first;
    },
    peak() {
      return this.isEmpty() ? null : this.nodes[0];
    },
    isEmpty() {
      return this.nodes.length === 0;
    }
  };
}

/**
               20

      14                28 

   10    15          24       32

4    11            21
*/

const tree = new Tree();

const n1 = Node(20);
tree.root = n1;

const n2 = tree.addLeft(14, n1);
const n3 = tree.addRight(28, n1);

const n4 = tree.addLeft(10, n2);
tree.addRight(15, n2);

const n5 = tree.addLeft(24, n3);
tree.addRight(32, n3);

tree.addLeft(4, n4);
tree.addRight(11, n4);

tree.addLeft(21, n5);

tree.levelOrder(tree.root, x => console.log(x));
//20,14,28,10,15,24,32,4,11,21