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Longest sequence of positive integers in an array
  • Difficulty Level : Easy
  • Last Updated : 29 May, 2018

Find the longest running positive sequence in an array.

Examples:

Input : arr[] = {1, 2, -3, 2, 3, 4, -6, 1, 
                     2, 3, 4, 5, -8, 5, 6}
Output :Index : 7, length : 5

Input : arr[] = {-3, -6, -1, -3, -8}
Output : No positive sequence detected.

A simple solution is to use two nested loops. In the outer loop, we look for positive elements. In the inner loop, we count number of positives starting with the positive element picked by outer loop. Time complexity of this solution is O(n^2).

An efficient solution is to use one single loop. We keep incrementing count while we see positive integers. We reset count to 0 after seeing a negative. Before resetting, we check if count is more than maximum.

C++




// CPP program to find longest running sequence
// of positive integers.
#include <bits/stdc++.h>
using namespace std;
  
// Prints longest sequence of positive integers in
// an array.
void getLongestSeq(int a[], int n)
{
    // Variables to keep track of maximum length and 
    // starting point of maximum length. And same 
    // for current length.
    int maxIdx = 0, maxLen = 0, currLen = 0, currIdx = 0;
  
    for (int k = 0; k < n; k++) {
        if (a[k] > 0) {
            currLen++;
  
            // New sequence, store
            // beginning index.
            if (currLen == 1) 
                currIdx = k;            
        }
        else {
            if (currLen > maxLen) {
                maxLen = currLen;
                maxIdx = currIdx;
            }
            currLen  = 0;
        }
    }
  
    if (maxLen > 0) 
        cout << "Length " << maxLen 
             << ", starting index " << maxIdx << endl;    
    else 
        cout << "No positive sequence detected." << endl;
      
    return;
}
  
// Driver code
int main()
{
    int arr[] = { 1, 2, -3, 2, 3, 4, -6, 
               1, 2, 3, 4, 5, -8, 5, 6 };
    int n = sizeof(arr) / sizeof(int);
    getLongestSeq(arr, n);
    return 0;
}


Java




// Java program to find longest running 
// sequence of positive integers
import java.io.*;
  
class GFG {
      
    // Prints longest sequence of 
    // positive integers in an array.
    static void getLongestSeq(int a[], int n)
    {
        // Variables to keep track of maximum
        // length and  starting point of 
        // maximum length. And same for current
        // length.
        int maxIdx = 0, maxLen = 0, currLen = 0, currIdx = 0;
      
        for (int k = 0; k < n; k++) 
        {
            if (a[k] > 0)
            {
                currLen++;
      
                // New sequence, store
                // beginning index.
                if (currLen == 1
                    currIdx = k;         
            }
            else
            {
                if (currLen > maxLen) 
                {
                    maxLen = currLen;
                    maxIdx = currIdx;
                }
                currLen = 0;
            }
        }
      
        if (maxLen > 0
        {
            System.out.print( "Length " + maxLen) ; 
            System.out.print( ",starting index " + maxIdx );
        }
        else
            System.out.println("No positive sequence detected.") ;
          
        return;
    }
      
    // Driver code
    public static void main (String[] args) 
    {
        int arr[] = { 1, 2, -3, 2, 3, 4, -6
                      1, 2, 3, 4, 5, -8, 5, 6 };
        int n = arr.length;
        getLongestSeq(arr, n);
      
    }
}
  
// This article is contributed by vt_m.


Python3




# Python code to find longest running 
# sequence of positive integers.
  
def getLongestSeq(a, n):
    maxIdx = 0
    maxLen = 0
    currLen = 0
    currIdx = 0
    for k in range(n):
        if a[k] > 0:
            currLen +=1
  
            # New sequence, store
            # beginning index.
            if currLen == 1:
                currIdx = k
        else:
            if currLen > maxLen:
                maxLen = currLen
                maxIdx = currIdx
            currLen = 0
              
    if maxLen > 0:
        print('Index : ',maxIdx,',Length : ',maxLen,)
    else:
        print("No positive sequence detected.")
  
# Driver code
arr = [ 1, 2, -3, 2, 3, 4, -6
        1, 2, 3, 4, 5, -8, 5, 6
n = len(arr)
getLongestSeq(arr, n)
  
# This code is contributed by "Abhishek Sharma 44"


C#




// C# program to find longest running
// sequence of positive integers.
using System;
  
class GFG {
      
    // Prints longest sequence of 
    // positive integers in an array.
    static void getLongestSeq(int []a, int n)
    {
          
        // Variables to keep track of maximum
        // length and starting point of 
        // maximum length. And same for current
        // length.
        int maxIdx = 0, maxLen = 0, currLen = 0,
            currIdx = 0;
      
        for (int k = 0; k < n; k++) 
        {
            if (a[k] > 0)
            {
                currLen++;
      
                // New sequence, store
                // beginning index.
                if (currLen == 1) 
                    currIdx = k;         
            }
            else
            {
                if (currLen > maxLen) 
                {
                    maxLen = currLen;
                    maxIdx = currIdx;
                }
                currLen = 0;
            }
        }
      
        if (maxLen > 0) 
        {
            Console.Write( "Length " + maxLen) ; 
            Console.WriteLine( ",starting index "
                                      + maxIdx );
        }
        else
            Console.WriteLine("No positive sequence"
                                   + " detected.") ;
          
        return;
    }
      
    // driver code
    public static void Main()
    {
        int []arr = { 1, 2, -3, 2, 3, 4, -6, 
                    1, 2, 3, 4, 5, -8, 5, 6 };
        int n = arr.Length;
        getLongestSeq(arr, n);
          
    
  
}
// This code is contributed by Sam007


PHP




<?php
// PHP program to find longest running
// sequence of positive integers.
  
// Prints longest sequence of positive 
// integers in an array.
function getLongestSeq($a, $n)
{
      
    // Variables to keep track 
    // of maximum length and 
    // starting point of maximum
    // length. And same for 
    // current length.
    $maxIdx = 0;
    $maxLen = 0;
    $currLen = 0;
    $currIdx = 0;
  
    for ($k = 0; $k < $n; $k++) {
        if ($a[$k] > 0) {
            $currLen++;
  
            // New sequence, store
            // beginning index.
            if ($currLen == 1) 
                $currIdx = $k;         
        }
        else {
            if ($currLen > $maxLen) {
                $maxLen = $currLen;
                $maxIdx = $currIdx;
            }
            $currLen = 0;
        }
    }
  
    if ($maxLen > 0) 
        echo "Length " . $maxLen
             ", starting index "
             $maxIdx ."\n"
    else
        echo "No positive sequence detected."."\n";
      
    return;
}
  
    // Driver Code
    $arr = array(1, 2, -3, 2, 3, 4, -6, 
                 1, 2, 3, 4, 5, -8, 5, 6);
    $n = count($arr);
    getLongestSeq($arr, $n);
      
// This code is contributed by Sam007
?>



Output:

Length 5, starting index 7

The algorithm is O(n) time and O(1) auxiliary space.

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