Java Program to find the number of occurrences of a given number using Binary Search approach

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This is a Java Program to find number of occurences of a given number using binary search approach. The time complexity of the following program is O (log n).

Here is the source code of the Java program to find number of occurences of a given number using binary search approach. The Java program is successfully compiled and run on a Windows system. The program output is also shown below.

  1. /*
  2.  *    Java Program to Find the Number of occurrences of a given Number using Binary Search approach
  3.  */
  4.  
  5. import java.util.Scanner;
  6.  
  7. public class NumberOfOccurences
  8. {
  9.     public static void main(String[] args) 
  10.     {
  11.         Scanner scan = new Scanner(System.in);
  12.         System.out.println("Enter number of elements in sorted array");
  13.         int N = scan.nextInt();
  14.         int[] arr = new int[ N ];
  15.         /* Accept N elements */
  16.         System.out.println("Enter "+ N +" sorted elements");
  17.         for (int i = 0; i < N; i++)
  18.             arr[i] = scan.nextInt();
  19.         System.out.println("Enter number to find occurences");
  20.         int num = scan.nextInt();
  21.  
  22.         int f = occur(arr, num);
  23.         if (f == -1)
  24.             System.out.println("No occurence");
  25.         else 
  26.             System.out.println("Occurences = "+ f);
  27.     }    
  28.     public static int occur(int[] arr, int num)
  29.     {
  30.         /* find first index */
  31.         int l1 = first(arr, num);
  32.         /* find last index */
  33.         int l2 = last(arr, num);
  34.         if (l1 == -1 || l2 == -1)
  35.             return -1;
  36.         return l2 - l1 + 1;
  37.     }
  38.     public static int first(int[] arr, int num)
  39.     {
  40.         if (arr[0] == num)
  41.             return 0;
  42.         int start = 0, end = arr.length - 1;
  43.         int mid = (start + end) / 2;
  44.         int flag = 0;
  45.         while (!(arr[mid] == num && arr[mid - 1] < arr[mid]))
  46.         {
  47.             if (start == end)
  48.             {
  49.                 flag = 1;
  50.                 break;
  51.             }
  52.             if (arr[mid] >= num)
  53.                 end = mid - 1;
  54.             if (arr[mid] < num)
  55.                 start = mid + 1;
  56.             mid = (start + end) / 2;
  57.         }
  58.         if (flag == 0)
  59.             return mid;
  60.         return -1;        
  61.     }
  62.     public static int last(int[] arr, int num)
  63.     {
  64.         if (arr[arr.length - 1] == num)
  65.             return arr.length - 1;
  66.         int start = 0, end = arr.length - 1;
  67.         int mid = (start + end) / 2;
  68.         int flag = 0;
  69.         while (!(arr[mid] == num && arr[mid + 1] > arr[mid]))
  70.         {
  71.             if (start == end)
  72.             {
  73.                 flag = 1;
  74.                 break;
  75.             }
  76.             if (arr[mid] > num)
  77.                 end = mid - 1;
  78.             if (arr[mid] <= num)
  79.                 start = mid + 1;
  80.             mid = (start + end) / 2;
  81.         }
  82.         if (flag == 0)
  83.             return mid;
  84.         return -1;        
  85.     }
  86. }

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Enter number of elements in sorted array
10
Enter 10 sorted elements
1 1 3 3 3 3 4 4 4 5
Enter number to find occurences
3
Occurences = 4
 
 
Enter number of elements in sorted array
10
Enter 10 sorted elements
1 1 3 3 3 3 4 4 4 5
Enter number to find occurences
5
Occurences = 1

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn | Youtube | Instagram | Facebook | Twitter