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;
     }
 }

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

@@ -1,10 +1,22 @@
 package com.thealgorithms.divideandconquer;
 
-// Java Program to Implement Strassen Algorithm
-// Class Strassen matrix multiplication
+// Java Program to Implement Strassen Algorithm for Matrix Multiplication
+
+/*
+ * Uses the divide and conquer approach to multiply two matrices.
+ * Time Complexity: O(n^2.8074) better than the O(n^3) of the standard matrix multiplication algorithm.
+ * Space Complexity: O(n^2)
+ * 
+ * This Matrix multiplication can be performed only on square matrices 
+ * where n is a power of 2. Order of both of the matrices are n × n.
+ * 
+ * Reference:
+ * https://www.tutorialspoint.com/design_and_analysis_of_algorithms/design_and_analysis_of_algorithms_strassens_matrix_multiplication.htm#:~:text=Strassen's%20Matrix%20multiplication%20can%20be,matrices%20are%20n%20%C3%97%20n.
+ * https://www.geeksforgeeks.org/strassens-matrix-multiplication/
+ */
+
 public class StrassenMatrixMultiplication {
 
-    // Method 1
     // Function to multiply matrices
     public int[][] multiply(int[][] A, int[][] B) {
         int n = A.length;
@@ -80,7 +92,6 @@ public class StrassenMatrixMultiplication {
         return R;
     }
 
-    // Method 2
     // Function to subtract two matrices
     public int[][] sub(int[][] A, int[][] B) {
         int n = A.length;
@@ -96,7 +107,6 @@ public class StrassenMatrixMultiplication {
         return C;
     }
 
-    // Method 3
     // Function to add two matrices
     public int[][] add(int[][] A, int[][] B) {
         int n = A.length;
@@ -112,9 +122,7 @@ public class StrassenMatrixMultiplication {
         return C;
     }
 
-    // Method 4
-    // Function to split parent matrix
-    // into child matrices
+    // Function to split parent matrix into child matrices
     public void split(int[][] P, int[][] C, int iB, int jB) {
         for (int i1 = 0, i2 = iB; i1 < C.length; i1++, i2++) {
             for (int j1 = 0, j2 = jB; j1 < C.length; j1++, j2++) {
@@ -123,9 +131,7 @@ public class StrassenMatrixMultiplication {
         }
     }
 
-    // Method 5
-    // Function to join child matrices
-    // into (to) parent matrix
+    // Function to join child matrices into (to) parent matrix
     public void join(int[][] C, int[][] P, int iB, int jB) {
         for (int i1 = 0, i2 = iB; i1 < C.length; i1++, i2++) {
             for (int j1 = 0, j2 = jB; j1 < C.length; j1++, j2++) {
@@ -134,49 +140,4 @@ public class StrassenMatrixMultiplication {
         }
     }
 
-    // Method 5
-    // Main driver method
-    public static void main(String[] args) {
-        System.out.println(
-            "Strassen Multiplication Algorithm Implementation For Matrix Multiplication :\n"
-        );
-
-        StrassenMatrixMultiplication s = new StrassenMatrixMultiplication();
-
-        // Size of matrix
-        // Considering size as 4 in order to illustrate
-        int N = 4;
-
-        // Matrix A
-        // Custom input to matrix
-        int[][] A = {
-            { 1, 2, 5, 4 },
-            { 9, 3, 0, 6 },
-            { 4, 6, 3, 1 },
-            { 0, 2, 0, 6 },
-        };
-
-        // Matrix B
-        // Custom input to matrix
-        int[][] B = {
-            { 1, 0, 4, 1 },
-            { 1, 2, 0, 2 },
-            { 0, 3, 1, 3 },
-            { 1, 8, 1, 2 },
-        };
-
-        // Matrix C computations
-        // Matrix C calling method to get Result
-        int[][] C = s.multiply(A, B);
-
-        System.out.println("\nProduct of matrices A and  B : ");
-
-        // Print the output
-        for (int i = 0; i < N; i++) {
-            for (int j = 0; j < N; j++) {
-                System.out.print(C[i][j] + " ");
-            }
-            System.out.println();
-        }
-    }
 }