occurrences in sorted array : done

Author: Ram swaroop

src/me/ramswaroop/arrays/OccurrencesInSortedArray.java

@@ -0,0 +1,85 @@
+package me.ramswaroop.arrays;
+
+/**
+ * Created by IntelliJ IDEA.
+ *
+ * @author: ramswaroop
+ * @date: 8/31/15
+ * @time: 2:52 PM
+ */
+public class OccurrencesInSortedArray {
+
+    /**
+     * Finds the occurrences of {@param k} in sorted array {@param a} in
+     * O(log n) time.
+     *
+     * @param a
+     * @param k
+     * @return
+     */
+    public static int getOccurrencesInSortedArray(int[] a, int k) {
+        int firstIndex = getFirstIndexOf(a, k, 0, a.length - 1);
+        // element not found
+        if (firstIndex == -1) {
+            return 0;
+        }
+        return getLastIndexOf(a, k, firstIndex, a.length - 1) - firstIndex + 1;
+    }
+
+    /**
+     * Returns the index of first occurrence of {@param n} in array {@param a}.
+     *
+     * @param a
+     * @param low
+     * @param high
+     * @param n
+     * @return
+     */
+    public static int getFirstIndexOf(int[] a, int n, int low, int high) {
+        if (low <= high) {
+            int mid = (low + high) / 2;
+            if (a[mid] == n && (mid == 0 || a[mid - 1] < n)) {
+                return mid;
+            } else if (a[mid] < n) {
+                return getFirstIndexOf(a, n, mid + 1, high);
+            } else {
+                return getFirstIndexOf(a, n, low, mid - 1);
+            }
+        } else {
+            return -1;
+        }
+    }
+
+    /**
+     * Returns the index of last occurrence of {@param n} in array {@param a}.
+     *
+     * @param a
+     * @param low
+     * @param high
+     * @param n
+     * @return
+     */
+    public static int getLastIndexOf(int[] a, int n, int low, int high) {
+        if (low <= high) {
+            int mid = (low + high) / 2;
+            if (a[mid] == n && (mid == a.length - 1 || a[mid + 1] > n)) {
+                return mid;
+            } else if (a[mid] <= n) {
+                return getLastIndexOf(a, n, mid + 1, high);
+            } else {
+                return getLastIndexOf(a, n, low, mid - 1);
+            }
+        } else {
+            return -1;
+        }
+    }
+
+    public static void main(String a[]) {
+        System.out.println(getOccurrencesInSortedArray(new int[]{1, 1, 2, 2, 2, 2, 3}, 1));
+        System.out.println(getOccurrencesInSortedArray(new int[]{1, 1, 1, 2, 2, 2, 2, 3}, 1));
+        System.out.println(getOccurrencesInSortedArray(new int[]{1, 1, 2, 2, 2, 2, 3}, 2));
+        System.out.println(getOccurrencesInSortedArray(new int[]{1, 1, 2, 2, 2, 2, 3}, 3));
+        System.out.println(getOccurrencesInSortedArray(new int[]{1, 1, 2, 2, 2, 2, 3}, 0));
+        System.out.println(getOccurrencesInSortedArray(new int[]{1, 1, 2, 2, 2, 2, 3}, 4));
+    }
+}