code done + unit tested

Author: Ram swaroop

src/me/ramswaroop/arrays/MissingAndRepeatingElements.java

@@ -0,0 +1,46 @@
+package me.ramswaroop.arrays;
+
+import java.util.Arrays;
+
+/**
+ * Created by IntelliJ IDEA.
+ *
+ * @author: ramswaroop
+ * @date: 9/7/15
+ * @time: 10:54 AM
+ */
+public class MissingAndRepeatingElements {
+
+    /**
+     * Finds two numbers in an unsorted array of size n with elements in range from
+     * 1 to n where one number from set {1, 2, …n} is missing and one number occurs
+     * twice in array.
+     *
+     * @param a
+     * @return an array where 1st element is the repeating element and 2nd is the missing one
+     */
+    public static int[] findMissingAndRepeatingElements(int[] a) {
+        int[] result = new int[2];
+        for (int i = 0; i < a.length; i++) {
+            // we use indices to mark already encountered numbers
+            if (a[Math.abs(a[i]) - 1] < 0) {
+                result[0] = Math.abs(a[i]); // repeating element
+            } else {
+                a[Math.abs(a[i]) - 1] = -a[Math.abs(a[i]) - 1];
+            }
+        }
+        // no. is +ve means its index wasn't encountered
+        for (int i = 0; i < a.length; i++) {
+            if (a[i] > 0) {
+                result[1] = i + 1; // missing element
+            }
+        }
+        return result;
+    }
+
+    public static void main(String a[]) {
+        System.out.println(Arrays.toString(findMissingAndRepeatingElements(new int[]{3, 1, 3})));
+        System.out.println(Arrays.toString(findMissingAndRepeatingElements(new int[]{4, 3, 6, 2, 1, 1})));
+        System.out.println(Arrays.toString(findMissingAndRepeatingElements(new int[]{4, 4, 6, 2, 5, 1})));
+    }
+}