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Maximum consecutive numbers present in an array

  • Difficulty Level : Easy
  • Last Updated : 29 Aug, 2021

Find the length of maximum number of consecutive numbers jumbled up in an array.
Examples: 
 

Input : arr[] = {1, 94, 93, 1000, 5, 92, 78};
Output : 3 
The largest set of consecutive elements is
92, 93, 94 

Input  : arr[] = {1, 5, 92, 4, 78, 6, 7};
Output : 4 
The largest set of consecutive elements is
4, 5, 6, 7

 

The idea is to use hashing. We traverse through the array and for every element, we check if it is the starting element of its sequence. If yes then by incrementing its value we search the set and increment the length. By repeating this for all elements, we can find the lengths of all consecutive sets in array. Finally we return length of the largest set.
 

C++




// CPP program to find largest consecutive numbers
// present in arr[].
#include <bits/stdc++.h>
using namespace std;
 
int findLongestConseqSubseq(int arr[], int n)
{
    /* We insert all the array elements into
       unordered set. */
    unordered_set<int> S;
    for (int i = 0; i < n; i++)
        S.insert(arr[i]);
 
    // check each possible sequence from the start
    // then update optimal length
    int ans = 0;
    for (int i = 0; i < n; i++) {
 
        // if current element is the starting
        // element of a sequence
        if (S.find(arr[i] - 1) == S.end()) {
 
            // Then check for next elements in the
            // sequence
            int j = arr[i];
 
            // increment the value of array element
            // and repeat search in the set
            while (S.find(j) != S.end())
                j++;
 
            // Update  optimal length if this length
            // is more. To get the length as it is
            // incremented one by one
            ans = max(ans, j - arr[i]);
        }
    }
    return ans;
}
 
// Driver code
int main()
{
    int arr[] = { 1, 94, 93, 1000, 5, 92, 78 };
    int n = sizeof(arr) / sizeof(int);
    cout << findLongestConseqSubseq(arr, n) << endl;
    return 0;
}

Java




// Java program to find largest consecutive
// numbers present in arr[].
import java.util.*;
 
class GFG
{
     
static int findLongestConseqSubseq(int arr[], int n)
{
    /* We insert all the array elements into
    unordered set. */
    HashSet<Integer> S = new HashSet<Integer>();
    for (int i = 0; i < n; i++)
        S.add(arr[i]);
 
    // check each possible sequence from the start
    // then update optimal length
    int ans = 0;
    for (int i = 0; i < n; i++)
    {
 
        // if current element is the starting
        // element of a sequence
        if(S.contains(arr[i]))
        {
 
            // Then check for next elements in the
            // sequence
            int j = arr[i];
 
            // increment the value of array element
            // and repeat search in the set
            while (S.contains(j))
                j++;
 
            // Update optimal length if this length
            // is more. To get the length as it is
            // incremented one by one
            ans = Math.max(ans, j - arr[i]);
        }
    }
    return ans;
}
 
// Driver code
public static void main(String[] args)
{
    int arr[] = {1, 94, 93, 1000, 5, 92, 78};
    int n = arr.length;
        System.out.println(findLongestConseqSubseq(arr, n));
}
}
 
// This code contributed by Rajput-Ji

Python3




# Python3 program to find largest consecutive
# numbers present in arr.
 
def findLongestConseqSubseq(arr, n):
    '''We insert all the array elements into unordered set.'''
 
    S = set();
    for i in range(n):
        S.add(arr[i]);
 
    # check each possible sequence from the start
    # then update optimal length
    ans = 0;
    for i in range(n):
         
        # if current element is the starting
        # element of a sequence
        if S.__contains__(arr[i]):
             
            # Then check for next elements in the
            # sequence
            j = arr[i];
             
            # increment the value of array element
            # and repeat search in the set
            while(S.__contains__(j)):
                j += 1;
 
            # Update optimal length if this length
            # is more. To get the length as it is
            # incremented one by one
            ans = max(ans, j - arr[i]);
    return ans;
 
# Driver code
if __name__ == '__main__':
    arr = [ 1, 94, 93, 1000, 5, 92, 78 ];
    n = len(arr);
    print(findLongestConseqSubseq(arr, n));
 
# This code is contributed by 29AjayKumar

C#




// C# program to find largest consecutive
// numbers present in arr[].
using System;
using System.Collections.Generic; public
 
class GFG
{
     
static int findLongestConseqSubseq(int []arr, int n)
{
    /* We insert all the array elements into
    unordered set. */
    HashSet<int> S = new HashSet<int>();
    for (int i = 0; i < n; i++)
        S.Add(arr[i]);
 
    // check each possible sequence from the start
    // then update optimal length
    int ans = 0;
    for (int i = 0; i < n; i++)
    {
 
        // if current element is the starting
        // element of a sequence
        if(S.Contains(arr[i]))
        {
 
            // Then check for next elements in the
            // sequence
            int j = arr[i];
 
            // increment the value of array element
            // and repeat search in the set
            while (S.Contains(j))
                j++;
 
            // Update optimal length if this length
            // is more. To get the length as it is
            // incremented one by one
            ans = Math.Max(ans, j - arr[i]);
        }
    }
    return ans;
}
 
// Driver code
public static void Main(String[] args)
{
    int []arr = {1, 94, 93, 1000, 5, 92, 78};
    int n = arr.Length;
    Console.WriteLine(findLongestConseqSubseq(arr, n));
}
}
 
// This code has been contributed by 29AjayKumar

Javascript




<script>
 
// JavaScript program to find largest consecutive numbers
// present in arr[].
 
 
function findLongestConseqSubseq(arr, n) {
    /* We insert all the array elements into
    unordered set. */
    let S = new Set();
    for (let i = 0; i < n; i++)
        S.add(arr[i]);
 
    // check each possible sequence from the start
    // then update optimal length
    let ans = 0;
    for (let i = 0; i < n; i++) {
 
        // if current element is the starting
        // element of a sequence
        if (!S.has(arr[i] - 1)) {
 
            // Then check for next elements in the
            // sequence
            let j = arr[i];
 
            // increment the value of array element
            // and repeat search in the set
            while (S.has(j))
                j++;
 
            // Update optimal length if this length
            // is more. To get the length as it is
            // incremented one by one
            ans = Math.max(ans, j - arr[i]);
        }
    }
    return ans;
}
 
// Driver code
 
let arr = [1, 94, 93, 1000, 5, 92, 78];
let n = arr.length;
document.write(findLongestConseqSubseq(arr, n) + "<br>");
 
</script>

Output:  

3

Time complexity : O(n2)
 

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