Add more tests (#3601)

src/main/java/com/thealgorithms/divideandconquer/BinaryExponentiation.java

@@ -1,22 +1,39 @@
 package com.thealgorithms.divideandconquer;
 
+// Java Program to Implement Binary Exponentiation (power in log n)
+
+/*
+ * Binary Exponentiation is a method to calculate a to the power of b.
+ * It is used to calculate a^n in O(log n) time.
+ * 
+ * Reference:
+ * https://iq.opengenus.org/binary-exponentiation/
+ */
+
 public class BinaryExponentiation {
 
-    public static void main(String args[]) {
-        System.out.println(calculatePower(2, 30));
-    }
-
-    // Function to calculate x^y
-    // Time Complexity: O(logn)
+    // recursive function to calculate a to the power of b
     public static long calculatePower(long x, long y) {
         if (y == 0) {
             return 1;
         }
         long val = calculatePower(x, y / 2);
-        val *= val;
-        if (y % 2 == 1) {
-            val *= x;
+        if (y % 2 == 0) {
+            return val * val;
         }
-        return val;
+        return val * val * x;
+    }
+
+    // iterative function to calculate a to the power of b
+    long power(long N, long M) {
+        long power = N, sum = 1;
+        while (M > 0) {
+            if ((M & 1) == 1) {
+                sum *= power;
+            }
+            power = power * power;
+            M = M >> 1;
+        }
+        return sum;
     }
 }