實作按下 +/- 要怎麼處理

AndroidCalculator/Calculator.swift

@@ -62,6 +62,16 @@ class Calculator {
       }
       formula.append(text)
       
+    case "+/-":
+      if ["+", "-", "×", "÷"].firstIndex(of: formula.last!) != nil {
+        formula.append("0")
+      }
+      if formula.last!.firstIndex(of: "-") != nil {
+        formula.append(String(formula.popLast()!.split(separator: "-")[0]))
+      } else {
+        formula.append("-" + formula.popLast()!)
+      }
+      
     default:
       print(formula)
     }