Symmetric Tree

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

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

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

But the following [1,2,2,null,3,null,3] is not:

    1
   / \
  2   2
   \   \
   3    3

Solution:

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