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Count Subarrays with Consecutive elements differing by 1

  • Difficulty Level : Easy
  • Last Updated : 07 May, 2021

Given an array arr[] of N integers. The task is to count the total number of subarrays of the given array such that difference between the consecutive elements in the subarrays is one. That is, for any index i  in the subarrays, arr[i+1] – arr[i] = 1.
Note: Do not consider subarrays with single element.
Examples: 
 

Input : arr[] = {1, 2, 3}
Output : 3
The subarrays are {1, 2}. {2, 3} and {1, 2, 3}

Input : arr[] = {1, 2, 3, 5, 6, 7}
Output : 6

 

Naive Approach: A simple approach is to run two nested loops and check every subarray and calculate the count of subarrays with consecutive elements differing by 1.
Efficient Approach: An efficient approach is to observe that in an array of length say K, total number of subarrays of size greater than 1 = (K)*(K-1)/2. 
So, the idea is to traverse the array by using two pointers to calculate subarrays with consecutive elements in a window of maximum length and then calculate all subarrays in that window using the above formula.
Below is the step by step algorithm: 
 

  • Take two pointers say fast and slow, for maintaining a window of consecutive elements.
  • Start traversing the array.
  • If elements differ by 1 increment only the fast pointer.
  • Else, calculate the length of current window between the indexes fast and slow.

Below is the implementation of the given approach: 
 

C++




// C++ program to count Subarrays with
// Consecutive elements differing by 1
 
#include <iostream>
using namespace std;
 
// Function to count Subarrays with
// Consecutive elements differing by 1
int subarrayCount(int arr[], int n)
{
    // Variable to store count of subarrays
    // whose consecutive elements differ by 1
    int result = 0;
 
    // Take two pointers for maintaining a
    // window of consecutive elements
    int fast = 0, slow = 0;
 
    // Traverse the array
    for (int i = 1; i < n; i++) {
 
        // If elements differ by 1
        // increment only the fast pointer
        if (arr[i] - arr[i - 1] == 1) {
            fast++;
        }
        else {
 
            // Calculate length of subarray
            int len = fast - slow + 1;
 
            // Calculate total subarrays except
            // Subarrays with single element
            result += len * (len - 1) / 2;
 
            // Update fast and slow
            fast = i;
            slow = i;
        }
    }
 
    // For last iteration. That is if array is
    // traversed and fast > slow
    if (fast != slow) {
        int len = fast - slow + 1;
        result += len * (len - 1) / 2;
    }
 
    return result;
}
 
// Driver Code
int main()
{
 
    int arr[] = { 1, 2, 3, 5, 6, 7 };
    int n = sizeof(arr) / sizeof(arr[0]);
 
    cout << subarrayCount(arr, n);
 
    return 0;
}

Java




// Java program to count Subarrays with
// Consecutive elements differing by 1
class cfg
{
 
// Function to count Subarrays with
// Consecutive elements differing by 1
static int subarrayCount(int arr[], int n)
{
    // Variable to store count of subarrays
    // whose consecutive elements differ by 1
    int result = 0;
 
    // Take two pointers for maintaining a
    // window of consecutive elements
    int fast = 0, slow = 0;
 
    // Traverse the array
    for (int i = 1; i < n; i++) {
 
        // If elements differ by 1
        // increment only the fast pointer
        if (arr[i] - arr[i - 1] == 1) {
            fast++;
        }
        else {
 
            // Calculate length of subarray
            int len = fast - slow + 1;
 
            // Calculate total subarrays except
            // Subarrays with single element
            result += len * (len - 1) / 2;
 
            // Update fast and slow
            fast = i;
            slow = i;
        }
    }
 
    // For last iteration. That is if array is
    // traversed and fast > slow
    if (fast != slow) {
        int len = fast - slow + 1;
        result += len * (len - 1) / 2;
    }
 
    return result;
}
 
// Driver Code
public static void main(String[] args)
{
 
    int arr[] = { 1, 2, 3, 5, 6, 7 };
    int n = arr.length;
 
    System.out.println(subarrayCount(arr, n));
 
}
}
//This code is contributed by Mukul Singh

Python3




# Python3 program to count Subarrays with
# Consecutive elements differing by 1
 
# Function to count Subarrays with
# Consecutive elements differing by 1
def subarrayCount(arr, n) :
     
    # Variable to store count of subarrays
    # whose consecutive elements differ by 1
    result = 0
 
    # Take two pointers for maintaining a
    # window of consecutive elements
    fast, slow = 0, 0
 
    # Traverse the array
    for i in range(1, n) :
 
        # If elements differ by 1
        # increment only the fast pointer
        if (arr[i] - arr[i - 1] == 1) :
            fast += 1
         
        else :
 
            # Calculate length of subarray
            length = fast - slow + 1
 
            # Calculate total subarrays except
            # Subarrays with single element
            result += length * (length - 1) // 2;
 
            # Update fast and slow
            fast = i
            slow = i
 
    # For last iteration. That is if array is
    # traversed and fast > slow
    if (fast != slow) :
        length = fast - slow + 1
        result += length * (length - 1) // 2;
     
    return result
 
# Driver Code
if __name__ == "__main__" :
 
    arr = [ 1, 2, 3, 5, 6, 7 ]
    n = len(arr)
 
    print(subarrayCount(arr, n))
 
# This code is contributed by Ryuga

C#




// C# program to count Subarrays with
// Consecutive elements differing by 1
using System;
class cfg
{
 
// Function to count Subarrays with
// Consecutive elements differing by 1
static int subarrayCount(int []arr, int n)
{
    // Variable to store count of subarrays
    // whose consecutive elements differ by 1
    int result = 0;
 
    // Take two pointers for maintaining a
    // window of consecutive elements
    int fast = 0, slow = 0;
 
    // Traverse the array
    for (int i = 1; i < n; i++) {
 
        // If elements differ by 1
        // increment only the fast pointer
        if (arr[i] - arr[i - 1] == 1) {
            fast++;
        }
        else {
 
            // Calculate length of subarray
            int len = fast - slow + 1;
 
            // Calculate total subarrays except
            // Subarrays with single element
            result += len * (len - 1) / 2;
 
            // Update fast and slow
            fast = i;
            slow = i;
        }
    }
 
    // For last iteration. That is if array is
    // traversed and fast > slow
    if (fast != slow) {
        int len = fast - slow + 1;
        result += len * (len - 1) / 2;
    }
 
    return result;
}
 
// Driver Code
public static void Main()
{
 
    int []arr = { 1, 2, 3, 5, 6, 7 };
    int n = arr.Length;
 
    Console.WriteLine(subarrayCount(arr, n));
 
}
}
//This code is contributed by inder_verma..

PHP




<?php
// PHP program to count Subarrays with
// Consecutive elements differing by 1
 
// Function to count Subarrays with
// Consecutive elements differing by 1
function subarrayCount($arr, $n)
{
    // Variable to store count of subarrays
    // whose consecutive elements differ by 1
    $result = 0;
 
    // Take two pointers for maintaining a
    // window of consecutive elements
    $fast = 0; $slow = 0;
 
    // Traverse the array
    for ($i = 1; $i < $n; $i++)
    {
 
        // If elements differ by 1
        // increment only the fast pointer
        if ($arr[$i] - $arr[$i - 1] == 1)
        {
            $fast++;
        }
        else
        {
 
            // Calculate length of subarray
            $len = $fast - $slow + 1;
 
            // Calculate total subarrays except
            // Subarrays with single element
            $result += $len * ($len - 1) / 2;
 
            // Update fast and slow
            $fast = $i;
            $slow = $i;
        }
    }
 
    // For last iteration. That is if array
    // is traversed and fast > slow
    if ($fast != $slow)
    {
        $len = $fast - $slow + 1;
        $result += $len * ($len - 1) / 2;
    }
 
    return $result;
}
 
// Driver Code
$arr = array(1, 2, 3, 5, 6, 7);
$n = sizeof($arr);
 
echo subarrayCount($arr, $n);
 
// This code is contributed
// by Akanksha Rai
?>

Javascript




<script>
 
// Javascript program to count Subarrays with
// Consecutive elements differing by 1
 
 
    // Function to count Subarrays with
    // Consecutive elements differing by 1
    function subarrayCount(arr , n) {
        // Variable to store count of subarrays
        // whose consecutive elements differ by 1
        var result = 0;
 
        // Take two pointers for maintaining a
        // window of consecutive elements
        var fast = 0, slow = 0;
 
        // Traverse the array
        for (i = 1; i < n; i++) {
 
            // If elements differ by 1
            // increment only the fast pointer
            if (arr[i] - arr[i - 1] == 1) {
                fast++;
            } else {
 
                // Calculate length of subarray
                var len = fast - slow + 1;
 
                // Calculate total subarrays except
                // Subarrays with single element
                result += len * (len - 1) / 2;
 
                // Update fast and slow
                fast = i;
                slow = i;
            }
        }
 
        // For last iteration. That is if array is
        // traversed and fast > slow
        if (fast != slow) {
            var len = fast - slow + 1;
            result += len * (len - 1) / 2;
        }
 
        return result;
    }
 
    // Driver Code
     
 
        var arr = [ 1, 2, 3, 5, 6, 7 ];
        var n = arr.length;
 
        document.write(subarrayCount(arr, n));
 
 
// This code contributed by aashish1995
 
</script>
Output: 



6

 

Time Complexity: O(N) 
Auxiliary Space: (1)
 

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