Added program for infix to postfix conversion using stack

Data-Structures/Stack/ToPostfix.js

@@ -0,0 +1,85 @@
+/*
+ * Author: Harinath-B (https://github.com/Harinath-B)
+ * Infix and POstfix notation explanation can be found in the following links:
+ * Wikipedia: https://en.wikipedia.org/wiki/Infix_notation / https://en.wikipedia.org/wiki/Reverse_Polish_notation
+ */
+
+class Stack {
+ constructor () {
+ this.stack = []
+ this.top = -1
+ }
+
+ // Adds a value to the end of the Stack
+ push (newValue) {
+ this.stack.push(newValue)
+ this.top++
+ }
+
+ // Returns and removes the last element of the Stack
+ pop () {
+ if (this.top !== -1) {
+ this.top--
+ return this.stack.pop()
+ }
+ throw new Error('Stack Underflow')
+ }
+
+ // Returns the number of elements in the Stack
+ length () {
+ return this.top
+ }
+
+ // Returns true if stack is empty, false otherwise
+ isEmpty () {
+ return this.top === -1
+ }
+
+ // Returns the last element without removing it
+ last () {
+ if (this.top !== -1) {
+ return this.stack[this.length()]
+ }
+ return null
+ }
+}
+
+const isAlNum = (c) => {
+ return c.match(/^[a-z0-9]+$/i) !== null
+}
+
+const priority = (op) => {
+ if (op === '+' || op === '-') {
+ return 1
+ } else if (op === '*' || op === '/') {
+ return 2
+ }
+ return 0
+}
+
+function ToPostfix (infix) {
+ let postfix = ''
+ const opStack = new Stack()
+ for (const c of infix) {
+ if (isAlNum(c)) {
+ postfix += c
+ } else if (c === '(') {
+ opStack.push(c)
+ } else if (c === ')') {
+ let x = ''
+ while ((x = opStack.pop()) !== '(') {
+ postfix += x
+ }
+ } else {
+ while (priority(opStack.last()) > priority(c)) {
+ postfix += opStack.pop()
+ } opStack.push(c)
+ }
+ }
+ while (opStack.top !== -1) {
+ postfix += opStack.pop()
+ }
+ return postfix
+}
+
+export { ToPostfix }

Data-Structures/Stack/test/ToPostfix.test.js

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+import { ToPostfix } from '../ToPostfix'
+
+describe('ToPostfix', () => {
+ it('Converting the given infix expression to postfix', () => {
+ const infix = 'a*b+(c-d/e)'
+ const postfix = ToPostfix(infix)
+ const expected = 'ab*cde/-+'
+ expect(postfix).toEqual(expected)
+ })
+
+ it('Testing empty input', () => {
+ const infix = ''
+ const postfix = ToPostfix(infix)
+ const expected = ''
+ expect(postfix).toEqual(expected)
+ })
+})