Maximum consecutive numbers present in an array

 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++
• Java
• Python3
• C#
• Javascript

// 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

Output

3

Time complexity : O(n2)
Space complexity: O(n)

Another approach: The idea is to sort the array. We will traverse through the array and check if the difference between the current element and the previous element is one or not. If the difference is one we will increment the count of the length of the current sequence. Otherwise, we will check if the count of the length of our current subsequence is greater than the length of our previously counted sequence. If it is, we will update our answer and then we will update the count to one to start counting the length of another sequence. By repeating this for all elements, we can find the lengths of all consecutive sequences in the array. Finally, we return the length of the largest sequence.

• C++
• Java
• Python3
• C#

// C# program to find largest consecutive numbers present // in arr. using System; using System.Collections;   public class GFG {     static int findLongestConseqSubseq(int[] arr, int n)   {       // The longest sequence in an empty array is, of     // course, 0, so we can simply return that.     if (n == 0) {       return 0;     }       // We will arrange array elements in ascending order     // using sort function.     Array.Sort(arr);       // check each possible sequence from the start then     // update optimal length     int ans = 1;     int count = 1;     for (int i = 1; i < n; i++) {       // For handling duplicate elements       if (arr[i] != arr[i - 1]) {         // if difference between current element and         // previous element is 1 then we want to         // update our current sequence count         if (arr[i] - arr[i - 1] == 1) {           count += 1;         }         // otherwise, we will update our count to         // zero to check for other sequences. before         // updating count value we have to check if         // current sequence length is more than our         // ans. if count > ans then we want to         // update our ans.         else {           ans = Math.Max(ans, count);           count = 1;         }       }     }       // To handle the case in which last element is     // present in longest sequence.     return Math.Max(ans, count);   }     static public void Main()   {       // Code     int[] arr = { 1, 94, 93, 1000, 5, 92, 78 };     int n = arr.Length;       // Function call     Console.Write(findLongestConseqSubseq(arr, n));   } }   // This code is contributed by lokeshmvs21.

Output

3

Time complexity : O(nlogn)

Space complexity: O(1)

Another approach: 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( no element whose value is less than the current element by one is present in the set ). If yes then by incrementing its value we search for other valid elements that could be present in the set and increment the length of the sequence accordingly. By repeating this for all elements, we can find the lengths of all consecutive sequences in the array. Finally, we return the length of the largest sequence

• Python3

# Python3 program to find largest consecutive # numbers present in arr.     def findLongestConseqSubseq(arr, n):     '''We insert all the array elements into set.'''       S = set(arr)       # check each possible sequence from the start     # then update optimal length     ans = 0     for e in arr:           # i contains current element of array         i = e         # count represents the length of current sequence         count = 1           # if current element is the starting         # element of a sequence         if i-1 not in S:             # Then check for next elements in the             # sequence             while i+1 in S:                 # increment the value of array element                 # and repeat search in the set                 i += 1                 count += 1               # Update optimal length if this length             # is more.             ans = max(ans, count)     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 sanjanasikarwar24

Output

3

Time complexity: O(n)

Space complexity: O(n)