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Task/Parsing-RPN-calculator-algorithm/Swift/parsing-rpn-calculator-algorithm.swift

@@ -0,0 +1,58 @@
+let opa = [
+    "^": (prec: 4, rAssoc: true),
+    "*": (prec: 3, rAssoc: false),
+    "/": (prec: 3, rAssoc: false),
+    "+": (prec: 2, rAssoc: false),
+    "-": (prec: 2, rAssoc: false),
+]
+
+func rpn(tokens: [String]) -> [String] {
+    var rpn : [String] = []
+    var stack : [String] = [] // holds operators and left parenthesis
+
+    for tok in tokens {
+        switch tok {
+        case "(":
+            stack += [tok] // push "(" to stack
+        case ")":
+            while !stack.isEmpty {
+                let op = stack.removeLast() // pop item from stack
+                if op == "(" {
+                    break // discard "("
+                } else {
+                    rpn += [op] // add operator to result
+                }
+            }
+        default:
+            if let o1 = opa[tok] { // token is an operator?
+                for op in stack.reverse() {
+                    if let o2 = opa[op] {
+                        if !(o1.prec > o2.prec || (o1.prec == o2.prec && o1.rAssoc)) {
+                            // top item is an operator that needs to come off
+                            rpn += [stack.removeLast()] // pop and add it to the result
+                            continue
+                        }
+                    }
+                    break
+                }
+
+                stack += [tok] // push operator (the new one) to stack
+            } else { // token is not an operator
+                rpn += [tok] // add operand to result
+            }
+        }
+    }
+
+    return rpn + stack.reverse()
+}
+
+func parseInfix(e: String) -> String {
+    let tokens = e.characters.split{ $0 == " " }.map(String.init)
+    return rpn(tokens).joinWithSeparator(" ")
+}
+
+var input : String
+
+input = "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3"
+"infix: \(input)"
+"postfix: \(parseInfix(input))"