Implemented recursive & iterative fibonacci (CoffeelessProgrammer#9 IP)

Author: CoffeelessProgrammer

Algorithms/README.md

@@ -1,7 +1,7 @@
 # Algorithms
 
 ## Core
-- [ ] Recursion
+- [X] Recursion
 - [ ] Dynamic Programming
 - [ ] Comparison Sorting
   - [ ] *Merge Sort*
@@ -16,4 +16,7 @@
   - [ ] Depth First Search (DFS)
 
 ## Resources
-- [Visualizing Data Structures & Algorithms](https://visualgo.net/en)
+- [Visualizing Data Structures & Algorithms](https://visualgo.net/en)
+
+### Recursion
+- [Tail Call Optimization: ES6](https://2ality.com/2015/06/tail-call-optimization.html)

Algorithms/Recursion/Basics.ts

@@ -1,8 +1,8 @@
 function inception(repeat: number): string {
   // 1. Base Case(s)
   if (repeat === 0) return 'Done!\n';         // Base Case(s) must be written first in method
-  // 2. Input Validation
-  if (repeat < 1) return 'Too small!\n';      // Input validation must occur after base case(s)
+  // 2?. Input Validation
+  if (repeat < 1) return 'Too small!\n';      // Input validation should occur after base case(s)
 
   console.log('Counter:', repeat);
 

Algorithms/Recursion/Fibonacci.ts

@@ -0,0 +1,106 @@
+// ---------------- Fibonacci Sequence ----------------
+// 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144...
+
+// Time Complexity: O(2^n)  Space Complexity: O(2^n)
+function fibonacciRecursive(nthElement: number): number | any {
+  // Base Cases
+  if (nthElement === 1) return 0;
+  if (nthElement === 2) return 1;
+  //Input Validation
+  if (nthElement < 1) return undefined;
+
+  // Recursive Call: The nth element equals the sum of the previous 2 elements.
+  return fibonacciRecursive(nthElement-2) + fibonacciRecursive(nthElement-1);
+}
+
+// Time Complexity: O(n)  Space Complexity: O(1)
+function fibonacciIterative(nthElement: number): number | undefined {
+  if (nthElement < 1) return undefined;
+  else nthElement = Math.floor(nthElement);
+
+  let previous = 0;
+  let current = 1;
+
+  if (nthElement === 1) return previous;
+  if (nthElement === 2) return current;
+
+  let fibonacci: number = 0;
+
+  for (let i = 3; i <= nthElement; ++i) {
+    fibonacci = previous + current;
+    previous = current;
+    current = fibonacci;
+  }
+
+  return fibonacci;
+}
+
+function validateFibonacciInput(nthElement: number) {
+  return nthElement>0;
+}
+
+function printFibonacciIterative(nthElement: number) {
+  console.log("Fibonacci Iterative", nthElement+':', fibonacciIterative(nthElement));
+}
+
+function printFibonacciRecursive(nthElement: number) {
+  if(!validateFibonacciInput(nthElement)) {
+    console.log("Input", nthElement, "is invalid -_-'");
+    return;
+  }
+  console.log("Fibonacci Recursive", nthElement+':', fibonacciRecursive(nthElement));
+}
+
+//---------------------------------------------------------------------
+// ----------                 MAIN PROGRAM                   ----------
+//---------------------------------------------------------------------
+if (import.meta.main) {
+
+  console.log('------------ Iterative ------------');
+  printFibonacciIterative(0);
+  printFibonacciIterative(1);
+  printFibonacciIterative(2);
+  printFibonacciIterative(3);
+  printFibonacciIterative(4);
+  printFibonacciIterative(5);
+  printFibonacciIterative(6);
+  printFibonacciIterative(9);
+  printFibonacciIterative(42);
+  printFibonacciIterative(50);
+  printFibonacciIterative(100);
+  console.log('\n------------ Recursive ------------');
+  printFibonacciRecursive(-1);
+  printFibonacciRecursive(0);
+  printFibonacciRecursive(1);
+  printFibonacciRecursive(2);
+  printFibonacciRecursive(3);
+  printFibonacciRecursive(4);
+  printFibonacciRecursive(5);
+  printFibonacciRecursive(7);
+  printFibonacciRecursive(8);
+  printFibonacciRecursive(42);  // Notice how much longer this takes than the iterative version
+  
+  // RUN:   deno run Algorithms/Recursion/Fibonacci.ts
+}
+
+// --------------------------- Terminal Output: ---------------------------
+// ------------ Iterative ------------
+// Fibonacci Iterative 0: undefined
+// Fibonacci Iterative 1: 0
+// Fibonacci Iterative 2: 1
+// Fibonacci Iterative 3: 1
+// Fibonacci Iterative 4: 2
+// Fibonacci Iterative 5: 3
+// Fibonacci Iterative 6: 5
+// Fibonacci Iterative 9: 21
+
+// ------------ Recursive ------------
+// Input -1 is invalid -_-'
+// Input 0 is invalid -_-'
+// Fibonacci Recursive 1: 0
+// Fibonacci Recursive 2: 1
+// Fibonacci Recursive 3: 1
+// Fibonacci Recursive 4: 2
+// Fibonacci Recursive 5: 3
+// Fibonacci Recursive 7: 8
+// Fibonacci Recursive 8: 13

Playground/Challenges/Recursion/ReverseString.ts

@@ -17,7 +17,7 @@
 - [X] Graphs
 
 ### Algorithms
-- [ ] Recursion
+- [X] Recursion
 - [ ] Dynamic Programming
 - [ ] Comparison Sorting
   - [ ] *Merge Sort*