https://twitter.com/CodeNewbies/status/1042900462226300936

BST Post Order

function Node(data) {
    this.data = data;
    this.left = null;
    this.right = null;
}

function BST() {
    this.root = null;
}

BST.prototype.push = function(data) {
    var current = this.root;

    if (!current) {
        this.root = new Node(data);
        return;
    }

    var node = new Node(data); 

    while (current) {
        if (data < current.data) {
            if (!current.left) {
                current.left = node;
                break;
            } else {
                current = current.left;
            }
        } else {
            if (!current.right) {
                current.right = node;
                break;
            } else {
                current = current.right;
            }
        }
    }
};

function postOrder(root) {
    var current = root,
        stack = [],
        completed = false;
    
    while (!completed) {
        if (current) {
            if (current.right) stack.push(current.right);
            
            stack.push(current);
            current = current.left;
        } else {
            if (stack.length) {
                current = stack.pop();
                if (current.right && stack[stack.length - 1] === current.right) {
                    stack.pop();
                    stack.push(current);
                    current = current.right;
                } else {
                    console.log(current.data);
                    current = null;
                }   
            } else {
                completed = true;
            }   
        }
    }
}

var binarySearchTree = new BST();
binarySearchTree.push(6);
binarySearchTree.push(4);
binarySearchTree.push(8);
binarySearchTree.push(2);
binarySearchTree.push(5);
binarySearchTree.push(7);
binarySearchTree.push(9);
binarySearchTree.push(1);
binarySearchTree.push(3);

postOrder(binarySearchTree.root);