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

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class Solution {
    public boolean isSymmetric(TreeNode root) {
        if(root == null)
            return true;

        return checkSame(root.left,root.right);
    }

    public boolean checkSame(TreeNode left, TreeNode right) {
        if(left == null && right == null)
            return true;
        if(left == null || right == null)
            return false;
        if(left.val == right.val)
            return checkSame(left.left, right.right) && checkSame(left.right,right.left);
        return false;
    }
}