symmetricBt

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
   public boolean isSymmetric(TreeNode root) {


       if (root == null) {
            return true;
        }

            return mirror(root.left,root.right); // check the tree

                                                                    //       1
    }                                                               //     /   \
                                                                    //   2       2
                                                                //      /
                                                            //         3   4    4   3
        public boolean mirror(TreeNode l, TreeNode r){          // l.l       ==       r.r
                                                                //       l.r == r.l
          if(l==null&&r==null){     // symmetric
                  return true;
          }else if(l!=null&&r!=null){
                  return l.val==r.val && mirror(l.left,r.right)&&   //
                    mirror(l.right,r.left);
          }


           return false;



        }
}