Add ES6 Examples to all chapters (#38)

* add ES6 example for binary search

* add ES6 example for selection sort

* add ES6 example for countdown

* add ES6 example for greet

* add ES6 example for factorial

* edit ES6 example for quicksort

* add ES6 example for loop sum

* add ES6 example for recursive sum

* add ES6 example for recursive count

* add ES6 example for recursive max

* add ES6 example for price of groceries

* add ES6 example for check voter

* add ES6 example for breadth-first search

* add ES6 example for dijkstras algorithm

* edit ES6 example for dijkstras algorithm

* edit ES6 example for set covering

* add ES6 example for longest common subsequence

03_recursion/ES6/01_countdown.js

@@ -0,0 +1,11 @@
+const countdown = (i) => {
+  console.log(i);
+  // base case
+  if (i <= 0) {
+    return null;
+  }
+  countdown(i - 1);
+  return null;
+};
+
+countdown(5);

03_recursion/ES6/02_greet.js

@@ -0,0 +1,12 @@
+const greet2 = name => console.log(`how are you, ${name}?`);
+
+const bye = () => console.log('ok bye!');
+
+const greet = (name) => {
+  console.log(`hello, ${name}!`);
+  greet2(name);
+  console.log('getting ready to say bye...');
+  bye();
+};
+
+greet('adit');

03_recursion/ES6/03_factorial.js

@@ -0,0 +1,8 @@
+const fact = (x) => {
+  if (x === 1) {
+    return 1;
+  }
+  return x * fact(x - 1);
+};
+
+console.log(fact(5));