Subtree of Another Tree

class Solution {
    public boolean isSameTree(TreeNode s, TreeNode t) {
        if (s == null && t == null) return true;
        if (s == null || t == null) return false;
        if (s.val != t.val) return false;
        return isSameTree(s.left, t.left) && isSameTree(s.right, t.right);
    }

    public boolean helper(TreeNode s, TreeNode t) {
        return s != null && (isSameTree(s, t) || helper(s.left, t) || helper(s.right, t));
    }

    public boolean isSubtree(TreeNode s, TreeNode t) {
        return helper(s, t);
    }
}