Reverse polish notation (#387)

* Added ReversePolishNotation

* Added link to Wikipedia

Maths/ReversePolishNotation.js

@@ -0,0 +1,35 @@
+// Wikipedia: https://en.wikipedia.org/wiki/Reverse_Polish_notation
+
+function calcRPN (expression) {
+  const operators = {
+    '+': (a, b) => a + b,
+    '-': (a, b) => a - b,
+    '*': (a, b) => a * b,
+    '/': (a, b) => b / a
+  }
+
+  const tokens = expression.split(' ')
+
+  const stack = []
+
+  tokens.forEach(token => {
+    const operator = operators[token]
+
+    if (typeof operator === 'function') {
+      const a = stack.pop()
+      const b = stack.pop()
+
+      const result = operator(a, b)
+
+      stack.push(result)
+    } else {
+      stack.push(parseFloat(token))
+    }
+  })
+
+  return stack.pop()
+}
+
+console.log(calcRPN('2 2 2 * +') === 6)
+console.log(calcRPN('2 2 + 2 *') === 8)
+console.log(calcRPN('6 9 7 + 2 / + 3 *') === 42)