二叉树的非递归遍历

public static void preOrderUnRecur(TreeNode root){
    if (root == null) return;
    Stack<TreeNode> stack = new Stack<>();
    stack.add(root);
    while (!stack.isEmpty()){
        TreeNode temp = stack.pop();
        System.out.print(temp.val+" ");
        if (temp.right != null){
            stack.add(temp.right);
        }
        if (temp.left != null){
            stack.add(temp.left);
        }
    }
}

    public static void inOrderUnRecur(TreeNode root){
        if (root == null) return;
        Stack<TreeNode> stack = new Stack<>();
        while (!stack.isEmpty() || root != null){
             if (root != null ){
                 stack.push(root);
                 root = root.left;
             }else {
                 TreeNode temp = stack.pop();
                 System.out.print(temp.val+" ");
                 root = temp.right;
             }
        }
    }

public static void posOrderUnRecur(TreeNode root){
    if (root == null) return;
    Stack<TreeNode> stack = new Stack<>();
    Stack<TreeNode> stack1 = new Stack<>();
    stack.add(root);
    while (!stack.isEmpty()){
        TreeNode temp = stack.pop();
        stack1.add(temp);
        if (temp.left != null){
            stack.add(temp.left);
        }
        if (temp.right != null){
            stack.add(temp.right);
        }
    }
    while (!stack1.isEmpty()){
        System.out.print(stack1.pop().val+" ");
    }
    System.out.println();
}