[LeetCode] Symmetric Tree

Thought:
Recursion.

Code:
public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }else {
            return symmetric(root.left, root.right);
        }
    }
    public boolean symmetric(TreeNode n1, TreeNode n2){
        if (n1 == null && n2 == null) {
            return true;
        }else{
            return n1 != null && n2 != null && n1.val == n2.val && symmetric(n1.left,n2.right) && symmetric(n1.right,n2.left);
        }
    }
}