Symmetric Tree

/**
 * Definition for binary tree
 * 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;
        if (root.left == null && root.right == null) return true;
        return isMirror(root.left, root.right);
    }
    
    public boolean isMirror(TreeNode n1, TreeNode n2) {
        if (n1 == null && n2 == null) return true;
        if (n1 == null && n2 != null) return false;
        if (n1 != null && n2 == null) return false;
        if (n1.val != n2.val) return false;
        return isMirror(n1.left, n2.right) && isMirror(n1.right, n2.left);
    }
}