Count of elements A[i] such that A[i] + 1 is also present in the Array

Given an integer array arr the task is to count the number of elements ‘A[i]’, such that A[i] + 1 is also present in the array.
Note: If there are duplicates in the array, count them separately.
Examples:

Input: arr = [1, 2, 3] 
Output: 2 
Explanation:
1 and 2 are counted cause 2 and 3 are in arr.
Input: arr = [1, 1, 3, 3, 5, 5, 7, 7] 
Output: 0 

Approach 1: Brute Force Solution
For all the elements in the array, return the total count after examining all elements 

• For current element x, compute x + 1, and search all positions before and after the current value for x + 1.
• If you find x + 1, add 1 to the total count

Below is the implementation of the above approach:

C++

// C++ program to count of elements
// A[i] such that A[i] + 1
// is also present in the Array
#include <bits/stdc++.h>
using namespace std;

// Function to find the countElements
int countElements(int* arr, int n)
{
    // Initialize count as zero
    int count = 0;

    // Iterate over each element
    for (int i = 0; i < n; i++) {

        // Store element in int x
        int x = arr[i];

        // Calculate x + 1
        int xPlusOne = x + 1;

        // Initialize found as false
        bool found = false;

        // Run loop to search for x + 1
        // after the current element
        for (int j = i + 1; j < n; j++) {
            if (arr[j] == xPlusOne) {
                found = true;
                break;
            }
        }

        // Run loop to search for x + 1
        // before the current element
        for (int k = i - 1;
             !found && k >= 0; k--) {
            if (arr[k] == xPlusOne) {
                found = true;
                break;
            }
        }

        // if found is true, increment count
        if (found == true) {
            count++;
        }
    }

    return count;
}

// Driver program
int main()
{
    int arr[] = { 1, 2, 3 };
    int n = sizeof(arr) / sizeof(arr[0]);

    // call countElements function on array
    cout << countElements(arr, n);

    return 0;
}

Java

// Java program to count of elements
// A[i] such that A[i] + 1
// is also present in the Array
import java.io.*;
import java.util.Arrays;

class GFG{

// Function to find the countElements
public static int countElements(int[] arr, int n)
{

    // Initialize count as zero
    int count = 0;

    // Iterate over each element
    for (int i = 0; i < n; i++)
    {

        // Store element in int x
        int x = arr[i];

        // Calculate x + 1
        int xPlusOne = x + 1;

        // Initialize found as false
        boolean found = false;

        // Run loop to search for x + 1
        // after the current element
        for (int j = i + 1; j < n; j++)
        {
            if (arr[j] == xPlusOne)
            {
                found = true;
                break;
            }
        }

        // Run loop to search for x + 1
        // before the current element
        for (int k = i - 1; !found && k >= 0; k--)
        {
            if (arr[k] == xPlusOne)
            {
                found = true;
                break;
            }
        }

        // If found is true, increment count
        if (found == true)
        {
            count++;
        }
    }
        return count;
}

// Driver code
public static void main (String[] args)
{
    int arr[] = { 1, 2, 3 };
    int n = arr.length;

    // Call countElements function on array
    System.out.println(countElements(arr, n));
}
}

//This code is contributed by shubhamsingh10

Python3

# Python3 program to count of elements
# A[i] such that A[i] + 1
# is also present in the Array

# Function to find the countElements
def countElements(arr,n):
    # Initialize count as zero
    count = 0

    # Iterate over each element
    for i in range(n):

        # Store element in int x
        x = arr[i]

        # Calculate x + 1
        xPlusOne = x + 1

        # Initialize found as false
        found = False

        # Run loop to search for x + 1
        # after the current element
        for j in range(i + 1,n,1):
            if (arr[j] == xPlusOne):
                found = True
                break

        # Run loop to search for x + 1
        # before the current element
        k = i - 1
        while(found == False and k >= 0):
            if (arr[k] == xPlusOne):
                found = True
                break
            k -= 1

        # if found is true, increment count
        if (found == True):
            count += 1

    return count

# Driver program
if __name__ == '__main__':
    arr = [1, 2, 3]
    n = len(arr)

    # call countElements function on array
    print(countElements(arr, n))

# This code is contributed by Surendra_Gangwar

C#

// C# program to count of elements
// A[i] such that A[i] + 1
// is also present in the Array
using System;

class GFG{

    // Function to find the countElements
    static int countElements(int[] arr, int n)
    {
        // Initialize count as zero
        int count = 0;

        // Iterate over each element
        for (int i = 0; i < n; i++) {

            // Store element in int x
            int x = arr[i];

            // Calculate x + 1
            int xPlusOne = x + 1;

            // Initialize found as false
            bool found = false;

            // Run loop to search for x + 1
            // after the current element
            for (int j = i + 1; j < n; j++) {
                if (arr[j] == xPlusOne) {
                    found = true;
                    break;
                }
            }

            // Run loop to search for x + 1
            // before the current element
            for (int k = i - 1;
                !found && k >= 0; k--) {
                if (arr[k] == xPlusOne) {
                    found = true;
                    break;
                }
            }

            // if found is true,
            // increment count
            if (found == true) {
                count++;
            }
        }

        return count;
    }

    // Driver program
    static public void Main ()
    {
        int[] arr = { 1, 2, 3 };
        int n = arr.Length;

        // call countElements function on array
        Console.WriteLine(countElements(arr, n));

    }
}

// This code is contributed by shubhamsingh10

Javascript

<script>
// JavaScript program to count of elements
// A[i] such that A[i] + 1
// is also present in the Array

// Function to find the countElements
function countElements(arr, n)
{
    // Initialize count as zero
    let count = 0;

    // Iterate over each element
    for (let i = 0; i < n; i++) {

        // Store element in int x
        let x = arr[i];

        // Calculate x + 1
        let xPlusOne = x + 1;

        // Initialize found as false
        let found = false;

        // Run loop to search for x + 1
        // after the current element
        for (let j = i + 1; j < n; j++)
        {
            if (arr[j] == xPlusOne)
            {
                found = true;
                break;
            }
        }

        // Run loop to search for x + 1
        // before the current element
        for (let k = i - 1;
            !found && k >= 0; k--) {
            if (arr[k] == xPlusOne) {
                found = true;
                break;
            }
        }

        // if found is true, increment count
        if (found == true) {
            count++;
        }
    }
    return count;
}

// Driver program
    let arr = [ 1, 2, 3 ];
    let n = arr.length;

    // call countElements function on array
    document.write(countElements(arr, n));

// This code is contributed by Surbhi Tyagi.
</script>

Output:

2

Time Complexity: In the above approach, for a given element, we check all other elements, So the time complexity is O(N*N) where N is no of elements. 
Auxiliary Space Complexity: In the above approach, we are not using any additional space, so Auxiliary space complexity is O(1).
Approach 2: Using Map

• For all elements in the array, say x, add x-1 to the map
• Again, for all elements in the array, say x, check if it exists in the map. If it exists, increment the counter
• Return the total count after examining all keys in the map

Below is the implementation of the above approach: 

C++

// C++ program to count of elements
// A[i] such that A[i] + 1
// is also present in the Array

#include <bits/stdc++.h>
using namespace std;

// Function to find the countElements
int countElements(vector<int>& arr)
{
    int size = arr.size();

    // Initialize result as zero
    int res = 0;

    // Create map
    map<int, bool> dat;

    // First loop to fill the map
    for (int i = 0; i < size; ++i) {
        dat[arr[i] - 1] = true;
    }

    // Second loop to check the map
    for (int i = 0; i < size; ++i) {
        if (dat[arr[i]] == true) {
            res++;
        }
    }
    return res;
}

// Driver program
int main()
{
    // Input Array
    vector<int> arr = { 1, 3, 2, 3, 5, 0 };

    // Call the countElements function
    cout << countElements(arr) << endl;

    return 0;
}

Java

// Java program to count of elements
// A[i] such that A[i] + 1 is
// also present in the Array
import java.util.*;

class GFG{

// Function to find the countElements
public static int countElements(int[] arr)
{
    int size = arr.length;

    // Initialize result as zero
    int res = 0;

    // Create map
    Map<Integer, Boolean> dat = new HashMap<>();

    // First loop to fill the map
    for(int i = 0; i < size; ++i)
    {
       dat.put((arr[i] - 1), true);
    }

    // Second loop to check the map
    for(int i = 0; i < size; ++i)
    {
       if (dat.containsKey(arr[i]) == true)
       {
           res++;
       }
    }
    return res;
}

// Driver code
public static void main(String[] args)
{

    // Input Array
    int[] arr = { 1, 3, 2, 3, 5, 0 };

    // Call the countElements function
    System.out.println(countElements(arr));

}
}

// This code is contributed by shad0w1947

Python3

# Python program to count of elements
# A[i] such that A[i] + 1
# is also present in the Array

# Function to find the countElements
def countElements(arr):

    size = len(arr)

    # Initialize result as zero
    res = 0

    # Create map
    dat={}

    # First loop to fill the map
    for i in range(size):
        dat[arr[i] - 1] = True

    # Second loop to check the map
    for i in range(size):
        if (arr[i] in dat):
            res += 1

    return res

# Driver program

# Input Array
arr =  [1, 3, 2, 3, 5, 0]

# Call the countElements function
print(countElements(arr))

# This code is contributed by shubhamsingh10

C#

// C# program to count of elements
// A[i] such that A[i] + 1 is
// also present in the Array
using System;
using System.Collections.Generic;
class GFG{

// Function to find the countElements
public static int countElements(int[] arr)
{
    int size = arr.Length;

    // Initialize result as zero
    int res = 0;

    // Create map
    Dictionary<int,
               Boolean> dat = new Dictionary<int,
                                             Boolean>();

    // First loop to fill the map
    for(int i = 0; i < size; ++i)
    {
       if(!dat.ContainsKey(arr[i] - 1))
          dat.Add((arr[i] - 1), true);
    }

    // Second loop to check the map
    for(int i = 0; i < size; ++i)
    {
       if (dat.ContainsKey(arr[i]) == true)
       {
           res++;
       }
    }
    return res;
}

// Driver code
public static void Main(String[] args)
{

    // Input Array
    int[] arr = { 1, 3, 2, 3, 5, 0 };

    // Call the countElements function
    Console.WriteLine(countElements(arr));
}
}

// This code is contributed by 29AjayKumar

Javascript

<script>
// javascript program to count of elements
// A[i] such that A[i] + 1 is
// also present in the Array

    // Function to find the countElements
    function countElements(arr) {
        var size = arr.length;

        // Initialize result as zero
        var res = 0;

        // Create map
        var dat = new Map();

        // First loop to fill the map
        for (i = 0; i < size; ++i) {
            dat.set((arr[i] - 1), true);
        }

        // Second loop to check the map
        for (i = 0; i < size; ++i) {
            if (dat.has(arr[i]) == true) {
                res++;
            }
        }
        return res;
    }

        // Input Array
        var arr = [ 1, 3, 2, 3, 5, 0 ];

        // Call the countElements function
        document.write(countElements(arr));

// This code is contributed by umadevi9616
</script>

Output:

3

Time Complexity: In the above approach, we iterate over the array twice. Once for filling the map and second time for checking the elements in the map, So the time complexity is O(N) where N is no of elements. 
Auxiliary Space Complexity: In the above approach, we are using an additional map which can contain N elements, so auxiliary space complexity is O(N).