Factorial of a Number

// Factorial: n! = n × (n-1) × (n-2) × ... × 2 × 1
// Example: 5! = 5 × 4 × 3 × 2 × 1 = 120

// Method 1: Using for loop
function factorialLoop(n) {
    if (n < 0) return "Invalid input";
    if (n === 0 || n === 1) return 1;
    
    let result = 1;
    for (let i = 2; i <= n; i++) {
        result *= i;
    }
    return result;
}

console.log("5! =", factorialLoop(5));
console.log("0! =", factorialLoop(0));
console.log("10! =", factorialLoop(10));

// Method 2: Using recursion
function factorialRecursive(n) {
    if (n < 0) return "Invalid input";
    if (n === 0 || n === 1) return 1;
    return n * factorialRecursive(n - 1);
}

console.log("\nUsing recursion:");
console.log("5! =", factorialRecursive(5));
console.log("7! =", factorialRecursive(7));

// Method 3: Using while loop
function factorialWhile(n) {
    if (n < 0) return "Invalid input";
    if (n === 0 || n === 1) return 1;
    
    let result = 1;
    let i = n;
    while (i > 1) {
        result *= i;
        i--;
    }
    return result;
}

console.log("\nUsing while:");
console.log("6! =", factorialWhile(6));

// Method 4: Using reduce
function factorialReduce(n) {
    if (n < 0) return "Invalid input";
    if (n === 0 || n === 1) return 1;
    
    return Array.from({length: n}, (_, i) => i + 1)
                .reduce((product, num) => product * num, 1);
}

console.log("\nUsing reduce:");
console.log("4! =", factorialReduce(4));