非递归方法遍历树（前序遍历、中序遍历及后续遍历）

public List<Integer> preorderTraversal(TreeNode root) {
     List<Integer> res = new ArrayList<>();
     if (root == null) return res;
     LinkedList<TreeNode> stack = new LinkedList<>();
     stack.push(root);
     while (!stack.isEmpty()) {
         TreeNode node = stack.pop();
         res.add(node.val);
         if (node.right != null) stack.push(node.right);
         if (node.left != null) stack.push(node.left);
     }
     return res;
 }

 public List<Integer> inorderTraversal(TreeNode root) {
     List<Integer> res = new ArrayList<>();
     if (root == null) return res;
     LinkedList<TreeNode> stack = new LinkedList<>();
     TreeNode currNode = root;

     while (currNode != null) {
         stack.push(currNode);
         currNode = currNode.left;
     }

     while (!stack.isEmpty()) {
         currNode = stack.pop();
         res.add(currNode.val);
         currNode = currNode.right;
         while (currNode != null) {
             stack.push(currNode);
             currNode = currNode.left;
         }
     }

     return res;
 }

 public List<Integer> postorderTraversal(TreeNode root) {
     List<Integer> res = new LinkedList<>();
     if (root == null) return res;
     LinkedList<TreeNode> nodeStack = new LinkedList<>();
     Set<TreeNode> nodeSet = new HashSet<>();

     TreeNode currNode = root;
     while (currNode != null) {
         nodeStack.push(currNode);
         currNode = currNode.left;
     }

     while (!nodeStack.isEmpty()) {
         currNode = nodeStack.peek();
         if (currNode.right != null && !nodeSet.contains(currNode.right)) {
             currNode = currNode.right;
             while (currNode != null) {
                 nodeStack.push(currNode);
                 currNode = currNode.left;
             }
         } else {
             currNode = nodeStack.pop();
             res.add(currNode.val);
             nodeSet.add(currNode);
         }
     }

     return res;
 }