Count Occurrences in Sorted Array

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Tue Feb 08 2022 13:34:04 GMT+0000 (Coordinated Universal Time)

Saved by @Uttam #java #gfg #geeksforgeeks #lecture #searching #sortedarray #countoccurrences

import java.util.*;
import java.io.*;

class GFG 
{ 
	static int firstOcc(int arr[], int n, int x)
	{
		int low = 0, high = n - 1;

		while(low <= high)
		{
			int mid = (low + high) / 2;

			if(x > arr[mid])
				low = mid + 1;

			else if(x < arr[mid])
				high = mid - 1;

			else
			{
				if(mid == 0 || arr[mid - 1] != arr[mid])
					return mid;

				else
					high = mid - 1;
			}

		}

		return -1;
	}

	static int lastOcc(int arr[], int n, int x)
	{
		int low = 0, high = n - 1;

		while(low <= high)
		{
			int mid = (low + high) / 2;

			if(x > arr[mid])
				low = mid + 1;

			else if(x < arr[mid])
				high = mid - 1;

			else
			{
				if(mid == n - 1 || arr[mid + 1] != arr[mid])
					return mid;

				else
					low = mid + 1;
			}

		}

		return -1;
	}
	
	
	static int countOcc(int arr[], int n, int x)
	{
		int first = firstOcc(arr, n, x);

		if(first == -1)
			return 0;
		else 
			return lastOcc(arr, n, x) - first + 1;
	}

	public static void main(String args[]) 
    {
        int arr[] = {10, 20, 20, 20, 40, 40}, n = 6;

		int x = 20;

		System.out.println(countOcc(arr, n, x));   // OUTPUT : 3

    } 
    
}
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Given a sorted array and an element x, we need to count occurrences of x in the array.