TheAlgorithms · DenizAltunkapan · Dec 17, 2025 · Dec 17, 2025 · Dec 17, 2025 · Dec 17, 2025
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+package com.thealgorithms.maths;
+
+/**
+ * In mathematics, the extended Euclidean algorithm is an extension to the
+ * Euclidean algorithm, and computes, in addition to the greatest common divisor
+ * (gcd) of integers a and b, also the coefficients of Bézout's identity, which
+ * are integers x and y such that ax + by = gcd(a, b).
+ *
+ * <p>
+ * For more details, see
+ * https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm
+ */
+public final class ExtendedEuclideanAlgorithm {
+
+    private ExtendedEuclideanAlgorithm() {
+    }
+
+    /**
+     * This method implements the extended Euclidean algorithm.
+     *
+     * @param a The first number.
+     * @param b The second number.
+     * @return An array of three integers:
+     *         <ul>
+     *         <li>Index 0: The greatest common divisor (gcd) of a and b.</li>
+     *         <li>Index 1: The value of x in the equation ax + by = gcd(a, b).</li>
+     *         <li>Index 2: The value of y in the equation ax + by = gcd(a, b).</li>
+     *         </ul>
+     */
+    public static long[] extendedGCD(long a, long b) {
+        if (b == 0) {
+            // Base case: gcd(a, 0) = a. The equation is a*1 + 0*0 = a.
+            return new long[] {a, 1, 0};
+        }
+
+        // Recursive call
+        long[] result = extendedGCD(b, a % b);
+        long gcd = result[0];
+        long x1 = result[1];
+        long y1 = result[2];
+
+        // Update coefficients using the results from the recursive call
+        long x = y1;
+        long y = x1 - a / b * y1;
+
+        return new long[] {gcd, x, y};
+    }
+}
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+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class ExtendedEuclideanAlgorithmTest {
+
+    /**
+     * Verifies that the returned values satisfy Bézout's identity: a*x + b*y =
+     * gcd(a, b)
+     */
+    private void verifyBezoutIdentity(long a, long b, long[] result) {
+        long gcd = result[0];
+        long x = result[1];
+        long y = result[2];
+        assertEquals(a * x + b * y, gcd, "Bézout's identity failed for gcd(" + a + ", " + b + ")");
+    }
+
+    @Test
+    public void testExtendedGCD() {
+        // Test case 1: General case gcd(30, 50) = 10
+        long[] result1 = ExtendedEuclideanAlgorithm.extendedGCD(30, 50);
+        assertEquals(10, result1[0], "Test Case 1 Failed: gcd(30, 50) should be 10");
+        verifyBezoutIdentity(30, 50, result1);
+
+        // Test case 2: Another general case gcd(240, 46) = 2
+        long[] result2 = ExtendedEuclideanAlgorithm.extendedGCD(240, 46);
+        assertEquals(2, result2[0], "Test Case 2 Failed: gcd(240, 46) should be 2");
+        verifyBezoutIdentity(240, 46, result2);
+
+        // Test case 3: Base case where b is 0, gcd(10, 0) = 10
+        long[] result3 = ExtendedEuclideanAlgorithm.extendedGCD(10, 0);
+        assertEquals(10, result3[0], "Test Case 3 Failed: gcd(10, 0) should be 10");
+        verifyBezoutIdentity(10, 0, result3);
+
+        // Test case 4: Numbers are co-prime gcd(17, 13) = 1
+        long[] result4 = ExtendedEuclideanAlgorithm.extendedGCD(17, 13);
+        assertEquals(1, result4[0], "Test Case 4 Failed: gcd(17, 13) should be 1");
+        verifyBezoutIdentity(17, 13, result4);
+
+        // Test case 5: One number is a multiple of the other gcd(100, 20) = 20
+        long[] result5 = ExtendedEuclideanAlgorithm.extendedGCD(100, 20);
+        assertEquals(20, result5[0], "Test Case 5 Failed: gcd(100, 20) should be 20");
+        verifyBezoutIdentity(100, 20, result5);
+    }
+}