實作按下 0 後怎麼處理

AndroidCalculator/Calculator.swift

@@ -27,6 +27,16 @@ class Calculator {
     case "AC":
       // 清空到剩下 0
       formula = ["0"]
+      
+    case "0":
+      if formula.last == "0" {
+        return
+      } else if ["+", "-", "×", "÷"].firstIndex(of: formula.last!) != nil {
+        formula.append(text)
+      } else {
+        formula.append(formula.popLast()! + text)
+      }
+      
     default:
       formula.append(text)
     }