[Tree] Tree Traversal Iterative Method

Inorder: LinkedList<>stack=.. while ( root != null || !stack.isEmpty()) {        if ( root != null )       {             stack.push(root);             root = root.left;        }        else // root ==null       {             root= stack.pop();             res.add(root.val);             root= root.right;       } }

Preorder: LinkedList<>stack=.. while (root!= null || !stack.isEmpty()) {         if (root!= null)         {               stack.push(root);               res.add(root.val);              root =root.left;           }         else // root == null         {                root = stack.pop();                root = root.right;         } }

Postorder: LinkedList<>stack=... TreeNode pre = new TreeNode(); while (root != null || !stack.isEmpty()) {        if (root!= null)        {               stack.push(root);               root = root.left;        }        else // root ==null        {               TreeNode peekNode = stack.peek();               //while (peekNode.right != null && peekNode.right != pre)                              //peekNode.right == null;               if ( peekNode.right != null && pre != peekNode.right)               {                      root = peekNode.right;                }               else { //peekNode.right ==null                      stack.pop();                      res.add(pre.val);                      //root = pre.right;                      pre = peekNode;               }         } } peekNode.right != null, right has a chilren, traverse first 2. pre!= peekNode.right, already traverse right children, traverse me parent(root) now