June 2018 Update

Task/Parsing-RPN-to-infix-conversion/Kotlin/parsing-rpn-to-infix-conversion.kotlin

@@ -0,0 +1,52 @@
+// version 1.2.0
+
+import java.util.Stack
+
+class Expression(var ex: String, val op: String = "", val prec: Int = 3) {
+
+    constructor(e1: String, e2: String, o: String) :
+        this("$e1 $o $e2", o, OPS.indexOf(o) / 2)
+
+    override fun toString() = ex
+
+    companion object {
+        const val OPS = "-+/*^"
+    }
+}
+
+fun postfixToInfix(postfix: String): String {
+    val expr = Stack<Expression>()
+    val rx = Regex("""\s+""")
+    for (token in postfix.split(rx)) {
+        val c = token[0]
+        val idx = Expression.OPS.indexOf(c)
+        if (idx != -1 && token.length == 1) {
+            val r = expr.pop()
+            val l = expr.pop()
+            val opPrec = idx / 2
+            if (l.prec < opPrec || (l.prec == opPrec && c == '^')) {
+                l.ex = "(${l.ex})"
+            }
+            if (r.prec < opPrec || (r.prec == opPrec && c != '^')) {
+                r.ex = "(${r.ex})"
+            }
+            expr.push(Expression(l.ex, r.ex, token))
+        }
+        else {
+            expr.push(Expression(token))
+        }
+        println("$token -> $expr")
+    }
+    return expr.peek().ex
+}
+
+fun main(args: Array<String>) {
+    val es = listOf(
+        "3 4 2 * 1 5 - 2 3 ^ ^ / +",
+        "1 2 + 3 4 + ^ 5 6 + ^"
+    )
+    for (e in es) {
+        println("Postfix : $e")
+        println("Infix : ${postfixToInfix(e)}\n")
+    }
+}