(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));
    }
}