chapter 9: recursion

Author: loiane

src/09-recursion/01-intro-recursion.js

@@ -0,0 +1,23 @@
+// src/09-recursion/01-intro-recursion.js
+
+// To understand recursion, one must first understand recursion
+const readline = require('readline').createInterface({
+  input: process.stdin,
+  output: process.stdout
+});
+
+function understandRecursion() {
+  readline.question('Do you understand recursion? (y/n) ', (answer) => {
+    if (answer.toLowerCase() === 'y') { // Base case
+      console.log("Excellent! You've grasped recursion.");
+      readline.close(); // Exit the program
+    } else {
+      console.log("Let's try again...");
+      understandRecursion(); // Recursive call
+    }
+  });
+}
+
+understandRecursion();
+
+// to see the output of this file use the command: node src/09-recursion/01-intro-recursion.js

src/09-recursion/02-factorial.js

@@ -0,0 +1,29 @@
+// src/09-recursion/02-factorial.js
+
+// iterative approach
+function factorialIterative(number) {
+  if (number < 0) {
+    return undefined;
+  }
+  let total = 1;
+  for (let n = number; n > 1; n--) {
+    total *= n;
+  }
+  return total;
+}
+
+console.log('5! =',factorialIterative(5)); // 5! = 120
+
+// recursive approach
+function factorial(number) {
+  // console.trace();
+  if (number < 0) { return undefined; }
+  if (number === 1 || number === 0) { // base case
+    return 1;
+  }
+  return number * factorial(number - 1);
+}
+
+console.log('Recursive 5! =',factorial(5)); // Recursive 5! = 120
+
+// to see the output of this file use the command: node src/09-recursion/02-factorial.js

src/09-recursion/03-callstack.js

@@ -0,0 +1,17 @@
+// src/09-recursion/03-callstack.js
+
+// stack overflow
+let count = 0;
+function recursiveFn() {
+  count++;
+  recursiveFn();
+}
+
+try {
+  recursiveFn();
+} catch (ex) {
+  console.log('count = ' + count + ' error: ' + ex);
+}
+
+
+// to see the output of this file use the command: node src/09-recursion/03-callstack.js

src/09-recursion/04-fibonacci.js

@@ -0,0 +1,54 @@
+// src/09-recursion/04-fibonacci.js
+
+// iterative approach
+function fibonacciIterative(n) {
+  if (n < 0) {
+    throw new Error('Input must be a non-negative integer');
+  }
+  if (n < 2) { return n; }
+
+  let prevPrev = 0;
+  let prev = 1;
+  let current;
+
+  for (let i = 2; i <= n; i++) { // n >= 2
+    current = prev + prevPrev; // f(n-1) + f(n-2)
+    prevPrev = prev;
+    prev = current;
+  }
+  
+  return current;
+}
+
+console.log('fibonacciIterative(2)', fibonacciIterative(2)); // 1
+console.log('fibonacciIterative(3)', fibonacciIterative(3)); // 2
+console.log('fibonacciIterative(4)', fibonacciIterative(4)); // 3
+console.log('fibonacciIterative(5)', fibonacciIterative(5)); // 5
+
+// recursive approach
+function fibonacci(n) {
+  if (n < 0) {
+    throw new Error('Input must be a non-negative integer');
+  }
+  if (n < 2) { return n; } // base case
+  return fibonacci(n - 1) + fibonacci(n - 2); // recursive case
+}
+
+console.log('fibonacci(5)', fibonacci(5)); // 5
+
+// memoization approach
+function fibonacciMemoization(n) {
+  if (n < 0) {
+    throw new Error('Input must be a non-negative integer');
+  }
+  const memo = [0, 1];
+  const fibonacci = (n) => {
+    if (memo[n] != null) return memo[n];
+    return memo[n] = fibonacci(n - 1) + fibonacci(n - 2);
+  };
+  return fibonacci(n);
+}
+
+console.log('fibonacciMemoization(5)', fibonacciMemoization(5)); // 5
+
+// to see the output of this file use the command: node src/09-recursion/04-fibonacci.js

src/09-recursion/leetcode/fibonacci-number.ts

@@ -0,0 +1,17 @@
+// https://leetcode.com/problems/fibonacci-number/description/
+
+function fib(n: number): number {
+  if (n < 2) { return n; }
+
+  let prevPrev = 0;
+  let prev = 1;
+  let current;
+
+  for (let i = 2; i <= n; i++) { // n >= 2
+    current = prev + prevPrev; // f(n-1) + f(n-2)
+    prevPrev = prev;
+    prev = current;
+  }
+  
+  return current;
+};

src/09-recursion/leetcode/power-of-two.ts

@@ -0,0 +1,43 @@
+// https://leetcode.com/problems/power-of-two/
+
+// function isPowerOfTwo(n: number): boolean {
+//   if (n <= 0) {
+//     return false;
+//   }
+
+//   return (n & (n - 1)) === 0;
+// }
+
+// Time complexity: O(1)
+// Space complexity: O(1)
+
+// Test cases
+
+
+// recursive solution
+function isPowerOfTwo(n: number): boolean {
+  if (n <= 0) { //  edge case for negative numbers
+    return false;
+  }
+  if (n % 2 !== 0) { // edge case for odd numbers
+    return false;
+  }
+
+  if (n === 1) { // base case
+    return true;
+  }
+
+  return isPowerOfTwo(n / 2);
+}
+
+// Time complexity: O(log n)
+// Space complexity: O(log n)
+
+// Test cases
+console.log(isPowerOfTwo(1)); // true
+console.log(isPowerOfTwo(16)); // true
+console.log(isPowerOfTwo(3)); // false
+console.log(isPowerOfTwo(4)); // true
+console.log(isPowerOfTwo(5)); // false
+console.log(isPowerOfTwo(0)); // false
+console.log(isPowerOfTwo(-1)); // false