101 Symmetric Tree

/**

* Definition for a binary tree node.

* public class TreeNode {

* int val;

* TreeNode left;

* TreeNode right;

* TreeNode(int x) { val = x; }

* }

*/

public class Solution {

public boolean isSymmetric(TreeNode root) {

if(root==null) return true;

return dfs(root.left, root.right);

}

public boolean dfs(TreeNode left, TreeNode right){

if(left==null&&right==null) return true;

if((left!=null&&right==null)||(left==null&&right!=null)) return false;

if(left.val==right.val){

return dfs(left.left,right.right)&&dfs(left.right,right.left);

}else{

return false;

}

}

}