Add GCD function to math directory

src/main/kotlin/math/Fibonacci.kt

@@ -0,0 +1,22 @@
+/**
+ * Returns the n-th Fibonacci number (0-based index) using iteration.
+ */
+fun fibonacci(n: Int): Long {
+    require(n >= 0) { "n must be non-negative" }
+    if (n == 0) return 0
+    if (n == 1) return 1
+    var a = 0L
+    var b = 1L
+    for (i in 2..n) {
+        val temp = a + b
+        a = b
+        b = temp
+    }
+    return b
+}
+
+fun main() {
+    println("Fibonacci(0) = ${fibonacci(0)}")   // 0
+    println("Fibonacci(1) = ${fibonacci(1)}")   // 1
+    println("Fibonacci(10) = ${fibonacci(10)}") // 55
+}

src/main/kotlin/math/GCD.kt

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+/**
+ * Computes the Greatest Common Divisor (GCD) of two integers using Euclid's algorithm.
+ */
+fun gcd(a: Int, b: Int): Int {
+    var x = a
+    var y = b
+    while (y != 0) {
+        val temp = y
+        y = x % y
+        x = temp
+    }
+    return kotlin.math.abs(x)
+}
+
+fun main() {
+    println("GCD of 12 and 18 is ${gcd(12, 18)}") // 6
+    println("GCD of 7 and 3 is ${gcd(7, 3)}")     // 1
+}

src/main/kotlin/math/IsPrime.kt

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+/**
+ * Returns true if the given number is prime, false otherwise.
+ */
+fun isPrime(n: Int): Boolean {
+    if (n < 2) return false
+    for (i in 2..kotlin.math.sqrt(n.toDouble()).toInt()) {
+        if (n % i == 0) return false
+    }
+    return true
+}
+
+fun main() {
+    println("7 is prime? ${isPrime(7)}")    // true
+    println("10 is prime? ${isPrime(10)}")  // false
+    println("13 is prime? ${isPrime(13)}")  // true
+}

src/main/kotlin/math/LCM.kt

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+/**
+ * Computes the Least Common Multiple (LCM) of two integers.
+ */
+fun lcm(a: Int, b: Int): Int {
+    return kotlin.math.abs(a * b) / gcd(a, b)
+}
+
+// Reusing gcd function from GCD.kt
+fun gcd(a: Int, b: Int): Int {
+    var x = a
+    var y = b
+    while (y != 0) {
+        val temp = y
+        y = x % y
+        x = temp
+    }
+    return kotlin.math.abs(x)
+}
+
+fun main() {
+    println("LCM of 12 and 18 is ${lcm(12, 18)}") // 36
+    println("LCM of 5 and 7 is ${lcm(5, 7)}")     // 35
+}

src/main/kotlin/math/nCr.kt

@@ -0,0 +1,17 @@
+/**
+ * Computes the number of combinations (n choose r)
+ */
+fun nCr(n: Int, r: Int): Long {
+    require(n >= 0 && r >= 0 && r <= n) { "Invalid n or r" }
+    var result = 1L
+    for (i in 1..r) {
+        result = result * (n - i + 1) / i
+    }
+    return result
+}
+
+fun main() {
+    println("C(5,2) = ${nCr(5,2)}")  // 10
+    println("C(10,3) = ${nCr(10,3)}") // 120
+    println("C(6,0) = ${nCr(6,0)}")  // 1
+}