前序、中序、后序、层次遍历皆可实现，我就实现一下前序和层次就好

前序遍历实现

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

层次遍历实现

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