226 Invert Binary Tree

nvert a binary tree.

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

to

     4
   /   \
  7     2
 / \   / \
9   6 3   1
题目含义：左右交换二叉树的节点值
方法一：DFS

    public TreeNode invertTree(TreeNode root) {
        if(root==null){
            return null;
        }
        TreeNode newNode = new TreeNode(root.val);
        newNode.left = invertTree(root.right);
        newNode.right = invertTree(root.left);
        return newNode;        
    }

方法二：BfS

    public TreeNode invertTree(TreeNode root) {
        if(root==null)return null;
        final Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()) {
            final TreeNode node = queue.poll();
            final TreeNode left = node.left;
            node.left = node.right;
            node.right = left;
            if(node.left != null) queue.offer(node.left);
            if(node.right != null) queue.offer(node.right);
        }
        return root;
    }