實作遇到加減乘除要怎麼處理

AndroidCalculator/Calculator.swift

@@ -56,6 +56,12 @@ class Calculator {
         formula.append(formula.popLast()! + text)
       }
       
+    case "+", "-", "×", "÷":
+      if ["+", "-", "×", "÷"].firstIndex(of: formula.last!) != nil {
+        formula.removeLast()
+      }
+      formula.append(text)
+      
     default:
       print(formula)
     }