Add code for chapter 3 and 4 in ts (#103)

03_recursion/ts/01_countdown.ts

@@ -0,0 +1,13 @@
+function countdown(i: number): null {
+    console.log(i);
+
+    if (i <= 0) {
+        return null;
+    }
+
+    countdown(i-1);
+
+    return null;
+}
+
+console.log(countdown(5));

03_recursion/ts/02_greet.ts

@@ -0,0 +1,16 @@
+function greet2(name: string): void {
+    console.log('how are you, ' + name + '?');
+}
+
+function bye(): void {
+    console.log('ok bye!');
+}
+
+function greet(name: string): void {
+    console.log('hello, ' + name + '!');
+    greet2(name);
+    console.log('getting ready to say bye...');
+    bye();
+}
+
+greet('adit');

03_recursion/ts/03_factorial.ts

@@ -0,0 +1,9 @@
+function fact(x: number): number {
+    if (x === 1) {
+        return x;
+    }
+
+    return x * fact(x - 1);
+}
+
+console.log(fact(5));