# Count pairs in array such that one element is reverse of another

Given an array arr[], the task is to count the pairs in the array such that elements are reverse of each other.

Examples:

Input: arr[] = { 16, 61, 12, 21, 25 }
Output: 2
Explanation:
The 2 pairs such that one number is the reverse of the other are {16, 61} and {12, 21}.

Input: arr[] = {10, 11, 12}
Output: 0

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach: The idea is to use nested loops to get all the possible pairs of numbers in the array. Then, for each pair, check whether an element is a reverse of another. If it is, then increase the required count by one. When all the pairs have been checked, return or print the count of such pair.

Below is the implementation of the above approach:

## C++

 `// C++ program to count the pairs in array ` `// such that one element is reverse of another ` `#include ` `using` `namespace` `std; ` ` `  `// Function to reverse the digits ` `// of the number ` `int` `reverse(``int` `num) ` `{ ` `    ``int` `rev_num = 0; ` ` `  `    ``// Loop to iterate till the number is ` `    ``// greater than 0 ` `    ``while` `(num > 0) { ` ` `  `        ``// Extract the last digit and keep ` `        ``// multiplying it by 10 to get the ` `        ``// reverse of the number ` `        ``rev_num = rev_num * 10 + num % 10; ` `        ``num = num / 10; ` `    ``} ` `    ``return` `rev_num; ` `} ` ` `  `// Function to find the pairs from the  ` `// such that one number is reverse of ` `// the other ` `int` `countReverse(``int` `arr[], ``int` `n) ` `{ ` `    ``int` `res = 0; ` ` `  `    ``// Iterate through all pairs ` `    ``for` `(``int` `i = 0; i < n; i++) ` `        ``for` `(``int` `j = i + 1; j < n; j++) ` ` `  `            ``// Increment count if one is ` `            ``// the reverse of other ` `            ``if` `(reverse(arr[i]) == arr[j]) { ` `                ``res++; ` `            ``} ` ` `  `    ``return` `res; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``int` `a[] = { 16, 61, 12, 21, 25 }; ` `    ``int` `n = ``sizeof``(a) / ``sizeof``(a); ` `    ``cout << countReverse(a, n); ` `    ``return` `0; ` `} `

## Java

 `// Java program to count the pairs in array ` `// such that one element is reverse of another ` ` `  `class` `Geeks { ` `    ``// Function to reverse the digits ` `    ``// of the number ` `    ``static` `int` `reverse(``int` `num) { ` `        ``int` `rev_num = ``0``; ` ` `  `        ``// Loop to iterate till the number is ` `        ``// greater than 0 ` `        ``while` `(num > ``0``) { ` ` `  `            ``// Extract the last digit and keep ` `            ``// multiplying it by 10 to get the ` `            ``// reverse of the number ` `            ``rev_num = rev_num * ``10` `+ num % ``10``; ` `            ``num = num / ``10``; ` `        ``} ` `        ``return` `rev_num; ` `    ``} ` ` `  `    ``// Function to find the pairs from the ` `    ``// such that one number is reverse of ` `    ``// the other ` `    ``static` `int` `countReverse(``int` `arr[], ``int` `n) { ` `        ``int` `res = ``0``; ` ` `  `        ``// Iterate through all pairs ` `        ``for` `(``int` `i = ``0``; i < n; i++) ` `            ``for` `(``int` `j = i + ``1``; j < n; j++) ` ` `  `                ``// Increment count if one is ` `                ``// the reverse of other ` `                ``if` `(reverse(arr[i]) == arr[j]) { ` `                    ``res++; ` `                ``} ` ` `  `        ``return` `res; ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `main(String[] args) { ` `        ``int` `a[] = { ``16``, ``61``, ``12``, ``21``, ``25` `}; ` `        ``int` `n = a.length; ` `        ``System.out.print(countReverse(a, n)); ` `    ``} ` `} ` ` `  `// This code is contributed by Rajnis09 `

## C#

 `// C# program to count the pairs in array ` `// such that one element is reverse of another ` `using` `System; ` `  `  `class` `GFG  ` `{ ` `  `  `// Function to reverse the digits ` `// of the number ` `static` `int` `reverse(``int` `num) ` `{ ` `    ``int` `rev_num = 0; ` `  `  `    ``// Loop to iterate till the number is ` `    ``// greater than 0 ` `    ``while` `(num > 0) { ` `  `  `        ``// Extract the last digit and keep ` `        ``// multiplying it by 10 to get the ` `        ``// reverse of the number ` `        ``rev_num = rev_num * 10 + num % 10; ` `        ``num = num / 10; ` `    ``} ` `    ``return` `rev_num; ` `} ` `  `  `// Function to find the pairs from the  ` `// such that one number is reverse of ` `// the other ` `static` `int` `countReverse(``int` `[]arr, ``int` `n) ` `{ ` `    ``int` `res = 0; ` `  `  `    ``// Iterate through all pairs ` `    ``for` `(``int` `i = 0; i < n; i++) ` `        ``for` `(``int` `j = i + 1; j < n; j++) ` `  `  `            ``// Increment count if one is ` `            ``// the reverse of other ` `            ``if` `(reverse(arr[i]) == arr[j]) { ` `                ``res++; ` `            ``} ` `  `  `    ``return` `res; ` `} ` `  `  `// Driver code  ` `public` `static` `void` `Main(String []arr)  ` `{ ` `    ``int` `[]a = { 16, 61, 12, 21, 25 }; ` `    ``int` `n = a.Length; ` `    ``Console.Write(countReverse(a, n)); ` `} ` `} ` ` `  `// This code is contributed by shivanisinghss2110 `

## Python3

 `# Python3 program to count the pairs in array ` `# such that one element is reverse of another ` ` `  `# Function to reverse the digits ` `# of the number ` `def` `reverse(num): ` `    ``rev_num ``=` `0` ` `  `    ``# Loop to iterate till the number is ` `    ``# greater than 0 ` `    ``while` `(num > ``0``): ` ` `  `        ``# Extract the last digit and keep ` `        ``# multiplying it by 10 to get the ` `        ``# reverse of the number ` `        ``rev_num ``=` `rev_num ``*` `10` `+` `num ``%` `10` `        ``num ``=` `num ``/``/` `10` ` `  `    ``return` `rev_num ` ` `  `# Function to find the pairs from the  ` `# such that one number is reverse of ` `# the other ` `def` `countReverse(arr,n): ` `    ``res ``=` `0` ` `  `    ``# Iterate through all pairs ` `    ``for` `i ``in` `range``(n): ` `        ``for` `j ``in` `range``(i ``+` `1``, n): ` ` `  `            ``# Increment count if one is ` `            ``# the reverse of other ` `            ``if` `(reverse(arr[i]) ``=``=` `arr[j]): ` `                ``res ``+``=` `1` ` `  `    ``return` `res ` ` `  `# Driver code ` `if` `__name__ ``=``=` `'__main__'``: ` `    ``a ``=`  `[``16``, ``61``, ``12``, ``21``, ``25``] ` `    ``n ``=`  `len``(a) ` `    ``print``(countReverse(a, n)) ` ` `  `# This code is contributed by Surendra_Gangwar `

Output:

```2
```

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