56.Invert Binary Tree-翻转二叉树

解析：递归

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