Length of the longest alternating subarray

Given an array of N including positive and negative numbers only. The task is to find the length of the longest alternating (means negative-positive-negative or positive-negative-positive) subarray present in the array.

Examples:

Input: a[] = {-5, -1, -1, 2, -2, -3}
Output: 3 
The subarray {-1, 2, -2} 

Input: a[] = {1, -5, 1, -5}
Output: 4

Approach: The following steps are followed to solve the problem:



  • Initially initialize cnt as 1.
  • Iterate among the array elements, check if it has an alternate sign.
  • Increase the cnt by 1 if it has a alternate sign.
  • If it does not has an alternate sign, then re-initialize cnt by 1.

Below is the implementation of the above approach:

C++

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// C++ program to find the longest alternating
// subarray in an array of N number
#include <bits/stdc++.h>
using namespace std;
  
// Function to find the longest subarray
int longestAlternatingSubarray(int a[], int n)
{
    // Length of longest alternating
    int longest = 1;
    int cnt = 1;
  
    // Iterate in the array
    for (int i = 1; i < n; i++) {
  
        // Check for alternate
        if (a[i] * a[i - 1] < 0) {
            cnt++;
            longest = max(longest, cnt);
        }
        else
            cnt = 1;
    }
    return longest;
}
  
/* Driver program to test above functions*/
int main()
{
    int a[] = { -5, -1, -1, 2, -2, -3 };
    int n = sizeof(a) / sizeof(a[0]);
  
    // Function to find the longest subarray
    cout << longestAlternatingSubarray(a, n);
    return 0;
}

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Python 3





# Python3 program to find the longest alternating

# subarray in an array of N number 


# Function to find the longest subarray

def longestAlternatingSubarray(a, n):


	# Length of longest alternating

	longest = 1

	cnt = 1


	# Iterate in the array

	i = 1

	while i < n: 

		# Check for alternate 
		if (a[i] * a[i - 1] < 0):
			cnt = cnt + 1
			longest = max(longest, cnt) 
		
		else:
			cnt = 1
		i = i + 1
	
	return longest 

# Driver Code
a = [ -5, -1, -1, 2, -2, -3 ]
n = len(a)

# Function to find the longest subarray 
print(longestAlternatingSubarray(a, n))

# This code is contributed 
# by shashank_sharma



[tabby title="C#"]

                











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// C# program to find the longest alternating 


// subarray in an array of N number 


using System; 


class GFG 


{ 


  


// Function to find the longest subarray 


static int longestAlternatingSubarray(int []a,  


                                      int n) 


{ 


    // Length of longest alternating 


    int longest = 1; 


    int cnt = 1; 


  


    // Iterate in the array 


    for (int i = 1; i < n; i++)  


    { 


  


        // Check for alternate 


        if (a[i] * a[i - 1] < 0) 


        { 


            cnt++; 


            longest = Math.Max(longest, cnt); 


        } 


        else


            cnt = 1; 


    } 


    return longest; 


} 


  


// Driver Code 


public static void Main() 


{ 


    int []a = { -5, -1, -1, 2, -2, -3 }; 


    int n = a.Length; 


  


    // Function to find the longest subarray 


    Console.Write(longestAlternatingSubarray(a, n)); 


} 


} 


  


// This code is contributed 


// by Akanksha Rai 



















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PHP














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<?php 


// PHP program to find the longest alternating 


// subarray in an array of N number 


  


// Function to find the longest subarray 


function longestAlternatingSubarray($a, $n) 


{ 


      


    // Length of longest alternating 


    $longest = 1; 


    $cnt = 1; 


  


    // Iterate in the array 


    for ($i = 1; $i < $n; $i++)  


    { 


  


        // Check for alternate 


        if ($a[$i] * $a[$i - 1] < 0)  


        { 


            $cnt++; 


            $longest = max($longest, $cnt); 


        } 


        else


            $cnt = 1; 


    } 


    return $longest; 


} 


  


// Driver Code 


$a = array(-5, -1, -1, 2, -2, -3 ); 


$n = sizeof($a); 


  


// Function to find the longest subarray 


echo longestAlternatingSubarray($a, $n); 


  


// This code is contributed by akt_mit 


?> 



















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Output:


3






          
          
          
          

            

    

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