diff --git a/AndroidCalculator/Calculator.swift b/AndroidCalculator/Calculator.swift
index 9656303..41a0982 100644
--- a/AndroidCalculator/Calculator.swift
+++ b/AndroidCalculator/Calculator.swift
@@ -9,14 +9,26 @@
 import Foundation
 
 class Calculator {
-  var process = ""
+  var formula = ["0"]
+  
+  var process: String {
+    get {
+      var result = ""
+      formula.forEach { (item) in
+        result.append(item)
+      }
+      return result
+    }
+  }
   var result = ""
   
   func click(_ text: String) {
-    if text == "AC" {
-      process = ""
-    } else {
-      process += text
+    switch text {
+    case "AC":
+      // 清空到剩下 0
+      formula = ["0"]
+    default:
+      formula.append(text)
     }
   }
 }