二叉树反转(Invert Binary Tree )

     思路：

       一次前序遍历

   

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