LC226— 翻转二叉树

     4
   /   \
  2     7
 / \   / \
1   3 6   9

     4
   /   \
  7     2
 / \   / \
9   6 3   1

    public TreeNode invertTree(TreeNode root) {
   
        if (root == null) {
   
            return null;
        }
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
   
            //取出第一个节点,进行交换
            int size = queue.size();
            for (int i = 0; i < size; i++) {
   
                TreeNode node = queue.poll();
                TreeNode temp = node.left;
                node.left = node.right;
                node.right = temp;

                if (node.left != null) {
   
                    queue.add(node.left);
                }
                if (node.right != null) {
   
                    queue.add(node.right);
                }
            }
        }
        return root;
    }

    public TreeNode invertTree1(TreeNode root) {
   
        //terminator 终止循环
        if (root == null) {
   
            return null;
        }
        //process 本层递归处理的语句
        //将左右交换
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        //drill down 向下一层次递归的的语句
        invertTree(root.left);
        invertTree(root.right);
        //reverse states 返回状态
        return root;
    }