adding an implementation of a queue using 2 stacks (#137)

* adding an implementation of a queue using 2 stacks

* Update QueueUsing2Stacks.js

Co-authored-by: vinayak <itssvinayak@gmail.com>

Author: hmizz

Data Structures/Queue/QueueUsing2Stacks.js

@@ -0,0 +1,64 @@
+// implementation of Queue using 2 stacks
+// contribution made by hamza chabchoub for a university project
+
+class Queue {
+  constructor () {
+    this.inputStack = []
+    this.outputStack = []
+  }
+
+  // Push item into the inputstack
+  enqueue (item) {
+    this.inputStack.push(item)
+  }
+
+  dequeue (item) {
+    // push all items to outputstack
+    this.outputStack = []
+    if (this.inputStack.length > 0) {
+      while (this.inputStack.length > 0) {
+        this.outputStack.push(this.inputStack.pop())
+      }
+    }
+    // display the top element of the outputstack
+    if (this.outputStack.length > 0) {
+      console.log(this.outputStack.pop())
+      // repush all the items to the inputstack
+      this.inputStack = []
+      while (this.outputStack.length > 0) {
+        this.inputStack.push(this.outputStack.pop())
+      }
+    }
+  }
+
+  // display elements of the inputstack
+  listIn () {
+    let i = 0
+    while (i < this.inputStack.length) {
+      console.log(this.inputStack[i])
+      i++
+    }
+  }
+
+  // display element of the outputstack
+  listOut () {
+    let i = 0
+    while (i < this.outputStack.length) {
+      console.log(this.outputStack[i])
+      i++
+    }
+  }
+}
+
+// testing
+
+const queue = new Queue()
+queue.enqueue(1)
+queue.enqueue(2)
+queue.enqueue(8)
+queue.enqueue(9)
+
+console.log(queue.dequeue())
+// ans = 1
+console.log(queue.dequeue())
+// ans = 2