Count number of elements between two given elements in array

• Difficulty Level : Easy
• Last Updated : 15 Apr, 2021

Given an unsorted array of n elements and also given two points num1 and num2. The task is to count number of elements occurs between the given points (excluding num1 and num2).
If there are multiple occurrences of num1 and num2, we need to consider leftmost occurrence of num1 and rightmost occurrence of num2.
Examples:

```Input : arr[] = {3 5 7 6 4 9 12 4 8}
num1 = 5
num2 = 4
Output : 5
Number of elements between leftmost occurrence
of 5 and rightmost occurrence of 4 is five.

Input : arr[] = {4, 6, 8, 3, 6, 2, 8, 9, 4}
num1 = 4
num2 = 4
Output : 7

Input : arr[] = {4, 6, 8, 3, 6, 2, 8, 9, 4}
num1 = 4
num2 = 10
Output : 0```

The solution should traverse array only once in all cases (when single or both elements are not present)

The idea is to traverse array from left and find first occurrence of num1. If we reach end, we return 0. Then we traverse from rightmost element and find num2. We traverse only till the point which is greater than index of num1. If we reach end, we return 0. If we found both elements, we return count using indexes of found elements.

CPP

 `// Program to count number of elements between``// two given elements.``#include ``using` `namespace` `std;` `// Function to count number of elements``// occurs between the elements.``int` `getCount(``int` `arr[], ``int` `n, ``int` `num1, ``int` `num2)``{``    ``// Find num1``    ``int` `i = 0;``    ``for` `(i = 0; i < n; i++)``        ``if` `(arr[i] == num1)``            ``break``;` `    ``// If num1 is not present or present at end``    ``if` `(i >= n-1)``        ``return` `0;` `    ``// Find num2``    ``int` `j;``    ``for` `(j = n-1; j >= i+1; j--)``        ``if` `(arr[j] == num2)``            ``break``;` `    ``// If num2 is not present``    ``if` `(j == i)``        ``return` `0;` `    ``// return number of elements between``    ``// the two elements.``    ``return` `(j - i - 1);``}` `// Driver Code``int` `main()``{``    ``int` `arr[] = { 3, 5, 7, 6, 4, 9, 12, 4, 8 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr[0]);``    ``int` `num1 = 5, num2 = 4;``    ``cout << getCount(arr, n, num1, num2);``    ``return` `0;``}`

Java

 `// Program to count number of elements``// between two given elements.``import` `java.io.*;` `class` `GFG``{``    ``// Function to count number of elements``    ``// occurs between the elements.``    ``static` `int` `getCount(``int` `arr[], ``int` `n,``                            ``int` `num1, ``int` `num2)``    ``{``        ``// Find num1``        ``int` `i = ``0``;``        ``for` `(i = ``0``; i < n; i++)``            ``if` `(arr[i] == num1)``                ``break``;``    ` `        ``// If num1 is not present``        ``// or present at end``        ``if` `(i >= n - ``1``)``            ``return` `0``;``    ` `        ``// Find num2``        ``int` `j;``        ``for` `(j = n - ``1``; j >= i + ``1``; j--)``            ``if` `(arr[j] == num2)``                ``break``;``    ` `        ``// If num2 is not present``        ``if` `(j == i)``            ``return` `0``;``    ` `        ``// return number of elements``        ``// between the two elements.``        ``return` `(j - i - ``1``);``    ``}``    ` `    ``// Driver program``    ``public` `static` `void` `main (String[] args)``    ``{``        ``int` `arr[] = { ``3``, ``5``, ``7``, ``6``, ``4``, ``9``, ``12``, ``4``, ``8` `};``        ``int` `n = arr.length;``        ``int` `num1 = ``5``, num2 = ``4``;``        ``System.out.println( getCount(arr, n, num1, num2));` `    ``}``}``// This code is contributed by vt_m`

Python3

 `# Python Program to count number of elements between``# two given elements.` `# Function to count number of elements``# occurs between the elements.``def` `getCount(arr, n, num1, num2):``    ` `    ``# Find num1``    ``for` `i ``in` `range``(``0``,n):``        ``if` `(arr[i] ``=``=` `num1):``            ``break``            ` `    ``#If num1 is not present or present at end``    ``if` `(i >``=` `n``-``1``):``        ``return` `0` `    ``# Find num2``    ``for` `j ``in` `range``(n``-``1``, i``+``1``, ``-``1``):``        ``if` `(arr[j] ``=``=` `num2):``            ``break` `    ``# If num2 is not present``    ``if` `(j ``=``=` `i):``        ``return` `0` `    ``# return number of elements between``    ``# the two elements.``    ``return` `(j ``-` `i ``-` `1``)` `# Driver Code``arr``=` `[ ``3``, ``5``, ``7``, ``6``, ``4``, ``9``, ``12``, ``4``, ``8` `]``n``=``len``(arr)``num1 ``=` `5``num2 ``=` `4``print``(getCount(arr, n, num1, num2))` `# This code is contributed by SHARIQ_JMI`

C#

 `// C# Program to count number of elements``// between two given elements.``using` `System;` `class` `GFG  {``    ` `    ``// Function to count number of elements``    ``// occurs between the elements.``    ``static` `int` `getCount(``int` `[]arr, ``int` `n,``                        ``int` `num1, ``int` `num2)``    ``{``        ` `        ``// Find num1``        ``int` `i = 0;``        ``for` `(i = 0; i < n; i++)``            ``if` `(arr[i] == num1)``                ``break``;``    ` `        ``// If num1 is not present``        ``// or present at end``        ``if` `(i >= n - 1)``            ``return` `0;``    ` `        ``// Find num2``        ``int` `j;``        ``for` `(j = n - 1; j >= i + 1; j--)``            ``if` `(arr[j] == num2)``                ``break``;``    ` `        ``// If num2 is not present``        ``if` `(j == i)``            ``return` `0;``    ` `        ``// return number of elements``        ``// between the two elements.``        ``return` `(j - i - 1);``    ``}``    ` `    ``// Driver Code``    ``public` `static` `void` `Main ()``    ``{``        ``int` `[]arr = {3, 5, 7, 6, 4, 9, 12, 4, 8};``        ``int` `n = arr.Length;``        ``int` `num1 = 5, num2 = 4;``        ``Console.WriteLine(getCount(arr, n, num1, num2));` `    ``}``}` `// This article is contributed by vt_m.`

PHP

 `= ``\$n` `- 1)``        ``return` `0;` `    ``// Find num2``    ``\$j``;``    ``for` `(``\$j` `= ``\$n` `- 1; ``\$j` `>= ``\$i` `+ 1; ``\$j``--)``        ``if` `(``\$arr``[``\$j``] == ``\$num2``)``            ``break``;` `    ``// If num2 is not present``    ``if` `(``\$j` `== ``\$i``)``        ``return` `0;` `    ``// return number of elements``    ``// betweenthe two elements.``    ``return` `(``\$j` `- ``\$i` `- 1);``}` `// Driver Code``\$arr` `= ``array``(3, 5, 7, 6, 4, 9, 12, 4, 8);``\$n` `= sizeof(``\$arr``);``\$num1` `= 5; ``\$num2` `= 4;``echo``(getCount(``\$arr``, ``\$n``, ``\$num1``, ``\$num2``));` `// This code is contributed by Ajit.``?>`

Javascript

 ``

Output:

`5`

How to handle multiple queries?
For handling multiple queries, we can use hashing and store leftmost and rightmost indexes for every element present in array. Once we have stored these, we can answer all queries in O(1) time.
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