一、递归后序遍历

    public static void postOrder(TreeNode root) {
        if (root == null) {
            return;
        }
        postOrder(root.getLeft());
        postOrder(root.getRight());
        System.out.println(root.getValue());
    }

二、非递归后序遍历

    public static void postOrderIterative(TreeNode root) {
        if (root == null) {
            return;
        }
        Stack<TreeNode> tempTreeNodeStack = new Stack<>();
        Stack<TreeNode> finalTreeNodeStack = new Stack<>();
        tempTreeNodeStack.push(root);
        while (!tempTreeNodeStack.isEmpty()) {
            TreeNode currentNode = tempTreeNodeStack.pop();
            finalTreeNodeStack.push(currentNode);
            if (currentNode.getLeft() != null) {
                tempTreeNodeStack.push(currentNode.getLeft());
            }
            if (currentNode.getRight() != null) {
                tempTreeNodeStack.push(currentNode.getRight());
            }
        }
        while (!finalTreeNodeStack.isEmpty()) {
            System.out.println(finalTreeNodeStack.pop().getValue());
        }
    }

采用了两个stack进行，先按照，根节点、右节点、左节点的顺序放入栈中，让再pop出来，最终便是左节点、右节点，根节点的后序遍历顺序。