Postorder binary tree traversal with a single stack

postorder_binary_traversal.js

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

let nodes = [
  new Node('a'), //0
  new Node('b'), //1
  new Node('c'), //2
  new Node('d'), //3
  new Node('e'), //4
  new Node('f'), //5
  new Node('g'),
  new Node('h'),
  new Node('i'),
];

console.log('======================= post order iterable');
function traversePostOrder_itr(node) {
  const stack = [];
  let popped = null;

  while(true) {
    if (node !== null) {
      stack.push(node);
      node = node.left;
    } else {
      if (stack.length === 0) {
        break;
      }
      // check the right node
      let top = stack[stack.length - 1];
      if (top.right === null || top.right === popped) {
        popped = stack.pop();
        console.log(popped.value);
      } else {
        node = top.right;
      }
    }
  }
}

traversePostOrder_itr(nodes[5]);