Added implement stack using queues

Author: ignacio-chiazzo

LeetcodeProblems/Implement_stack_using_queues.js

@@ -0,0 +1,98 @@
+/*
+Implement Stack using Queues
+URL: https://leetcode.com/problems/implement-stack-using-queues/description/
+
+Implement the following operations of a stack using queues.
+
+push(x) -- Push element x onto stack.
+pop() -- Removes the element on top of the stack.
+top() -- Get the top element.
+empty() -- Return whether the stack is empty.
+Example:
+
+MyStack stack = new MyStack();
+
+stack.push(1);
+stack.push(2);  
+stack.top();   // returns 2
+stack.pop();   // returns 2
+stack.empty(); // returns false
+Notes:
+
+You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
+Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
+You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).
+*/
+
+
+
+class MyStack  {
+  constructor() {
+    this.q1 = [];
+    this.q2 = [];
+  };
+
+  push(elem) {
+    if(this.q1.length > 0) {
+      this.q1.push(elem);
+    } else {
+      this.q2.push(elem); 
+    }
+  };
+
+  pop() {
+    if(this.q1.length === 0 && this.q2.length === 0)
+      return null;
+
+    if(this.q1.length > 0) {
+      while(this.q1.length > 1) {
+        var elem = this.q1.shift();
+        this.q2.push(elem);
+      }
+      return this.q1.shift();
+    } else {
+      while(this.q2.length > 1) {
+        var elem = this.q2.shift();
+        this.q1.push(elem);
+      }
+      return this.q2.shift();
+    }
+  };
+
+  top() {
+    if(this.q1.length === 0 && this.q2.length === 0)
+      return null;
+
+    if(this.q1.length > 0) {
+      var elem = this.pop();
+      this.q2.push(elem);
+      return elem;
+    } else {
+      var elem = this.pop();
+      this.q1.push(elem);
+      return elem;
+    }
+  };
+
+  empty() {
+    return this.q1.length == 0 && this.q2.length === 0;
+  };
+}
+
+var main = function() {
+
+var myStack = new MyStack();
+  myStack.push(4);
+  myStack.push(3);
+  myStack.push(2);
+  myStack.push(1);
+  console.log(myStack.pop());
+  console.log(myStack.top());
+  console.log(myStack.push(1));
+  console.log(myStack.top());
+  console.log(myStack.pop());
+  console.log(myStack.pop());
+  console.log(myStack.pop());
+}
+
+module.exports.main = main;