Stopping rotation at 90° and -90°/270°.

I have an Transform that i am rotating backwards through touch commands on android. The rotation itself works fine. What I want to happen though is when it reaches 270 / -90 that it cannot rotate anymore and the same if it reaches 90 degrees the opposite direction. Upon reaching those points i want it to reset back to its zero degree rotation. Any help would be greatly appreciated. Thanks in advanced.

My code: [code]using UnityEngine; using System.Collections;

public class Pivot : MonoBehaviour {

 public float speed = 10.0f;
 public Vector2 touchDeltaPosition;
 
 
 // Use this for initialization
 void Start () {
 
 }
 
 // Update is called once per frame
 void Update () {
     if (Input.touchCount > 0 && Input.GetTouch(0).phase == TouchPhase.Moved) {
         touchDeltaPosition = Input.GetTouch(0).deltaPosition;
     } 
 }
 
 void LateUpdate () {
     
     
     if(Input.touchCount > 0 && touchDeltaPosition.x <= 0) {
         if(transform.rotation.z >= 270  || transform.rotation.z <= 90) {
             transform.Rotate(0, 0, -speed );
         }
         else {
             transform.Rotate(0, 0, 270);
         }

     }
 }

}[/code]