翻转二叉树

https://leetcode-cn.com/problems/invert-binary-tree/submissions/
递归法：

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null) return root;
        TreeNode left = invertTree(root.left);
        TreeNode right = invertTree(root.right);
        root.left = right;
        root.right = left;

        return root;
    }
}

层序遍历法：

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null) return null; 
        Queue<TreeNode> que =  new LinkedList();
        que.offer(root);

        while(!que.isEmpty()){
            TreeNode poll = que.poll();
            TreeNode temp = poll.right;
            poll.right = poll.left;
            poll.left = temp;

            if(poll.left != null){
                que.offer(poll.left);
            }
            if(poll.right != null){
                que.offer(poll.right);
            }
        }
        return root;
    }
}