Added sort a stack problem and solution. (knaxus#85)

* add solution to sort a stack problem

* update README.md

* add time compexity

Author: SumeetHaryani

README.md

@@ -29,6 +29,8 @@ Collection of interview questions with Unit Tests. Problems includes Data Struct
   - [Postfix Expression Evaluation](src/_DataStructures_/Stack/postfix-expression-evaluation)
   - [Remove Consecutive Repeated Digits](src/_DataStructures_/Stack/remove-consecutive-repeated-digits)
   - [Implement 2 Stacks using Single Array](src/_DataStructures_/Stack/2-stacks-using1-array)
+  - [Sort a Stack](src/_DataStructures_/Stack/sort-a-stack)
+
 
 - [Queue](src/_DataStructures_/Queue)
 

src/_DataStructures_/Stack/sort-a-stack/index.js

@@ -0,0 +1,36 @@
+/**
+ * Sort a stack with the help of a temporary stack.
+ * Input:[1,10,21,3,9,-11,32]
+ * Output:[32,21,10,9,3,1,-11]
+ * Time Complexity:O(N^2) 
+*/
+const Stack = require('../index');
+
+function sortStack(stack) {
+    const tempStack = new Stack();
+    while (!stack.isEmpty()) {
+        //pop the first element from stack
+        let temp = stack.pop();
+        //for ascending order (tempStack.peek() < temp)
+        while (!tempStack.isEmpty() && tempStack.peek() > temp) {
+            stack.push(tempStack.pop());
+        }
+        //push the first element(temp) onto tempStack if tempStack.peek()<temp
+        tempStack.push(temp);
+    }
+    return tempStack;
+}
+/*
+const s = new Stack();
+
+s.push(1);
+s.push(10);
+s.push(21);
+s.push(3);
+s.push(9);
+s.push(-11);
+s.push(32);
+console.log(s.data);
+const s1 = sortStack(s);
+console.log(s1.data);
+*/