(Tree) 101. Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

1
/ \
2   2
/ \ / \
3  4 4  3

But the following is not:

1
/ \
2   2
\   \
3    3

/**
* 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) {
return isSymmetric(root,root);
}
public boolean isSymmetric(TreeNode t1,TreeNode t2) {
if(t1==null && t2==null)  return true;
if(t1==null || t2==null)  return false;
return(t1.val==t2.val && isSymmetric(t1.left,t2.right) && isSymmetric(t1.right,t2.left));
}
}