Longest Continuous Sequence of Equal Integers From List Python

Given an array of integers, find the length of the longest sub-sequence such that elements in the subsequence are consecutive integers, the consecutive numbers can be in any order.

Examples:

Input: arr[] = {1, 9, 3, 10, 4, 20, 2}
Output: 4
Explanation: The subsequence 1, 3, 4, 2 is the longest subsequence of consecutive elements

Input: arr[] = {36, 41, 56, 35, 44, 33, 34, 92, 43, 32, 42}
Output: 5
Explanation: The subsequence 36, 35, 33, 34, 32 is the longest subsequence of consecutive elements.

Naive Approach:

The idea is to first sort the array and find the longest subarray with consecutive elements. After sorting the array and removing the multiple occurrences of elements, run a loop and keep a count and max (both initially zero). Run a loop from start to end and if the current element is not equal to the previous (element+1) then set the count to 1 else increase the count. Update max with a maximum of count and max.

 Illustration:

Input: arr[] = {1, 9, 3, 10, 4, 20, 2}

First sort the array to arrange them in consecutive fashion.
arr[] = {1, 2, 3, 4, 9, 10, 20}

Now, store the distinct elements from the sorted array.
dist[] = {1, 2, 3, 4, 9, 10, 20}

Initialise countConsecutive with 0 which will increment when arr[i] == arr[i – 1] + 1 is true otherwise countConsecutive will re-initialise by 1.

Maintain a variable ans to store maximum count of consecutive elements so far.

At i = 0:

• as i is 0 then re-initialise countConsecutive by 1.
• ans = max(ans, countConsecutive) = max(0, 1) = 1

At i = 1:

• check if (dist[1] == dist[0] + 1) = (2 == 1 + 1) = true
• as the above condition is true, therefore increment countConsecutive by 1
  • countConsecutive = countConsecutive + 1 = 1 + 1 = 2
• ans = max(ans, countConsecutive) = max(1, 2) = 1

At i = 2:

• check if (dist[2] == dist[1] + 1) = (3 == 2 + 1) = true
• as the above condition is true, therefore increment countConsecutive by 1
  • countConsecutive = countConsecutive + 1 = 2 + 1 = 3
• ans = max(ans, countConsecutive) = max(2, 3) = 3

At i = 3:

• check if (dist[3] == dist[2] + 1) = (4 == 3 + 1) = true
• as the above condition is true, therefore increment countConsecutive by 1
  • countConsecutive = countConsecutive + 1 = 3 + 1 = 4
• ans = max(ans, countConsecutive) = max(3, 4) = 4

At i = 4:

• check if (dist[4] == dist[3] + 1) = (9 != 4 + 1) = false
• as the above condition is false, therefore re-initialise countConsecutive by 1
  • countConsecutive = 1
• ans = max(ans, countConsecutive) = max(4, 1) = 4

At i = 5:

• check if (dist[5] == dist[4] + 1) = (10 == 9 + 1) = true
• as the above condition is true, therefore increment countConsecutive by 1
  • countConsecutive = countConsecutive + 1 = 1 + 1 = 2
• ans = max(ans, countConsecutive) = max(4, 2) = 4

At i = 6:

• check if (dist[6] == dist[5] + 1) = (20 != 10 + 1) = false
• as the above condition is false, therefore re-initialise countConsecutive by 1
  • countConsecutive = 1
• ans = max(ans, countConsecutive) = max(4, 1) = 4

Therefore the longest consecutive subsequence is {1, 2, 3, 4}
Hence, ans is 4.

Follow the steps below to solve the problem:

• Initialise ans and countConsecutive with 0.
• Sort the arr[].
• Store the distinct elements in dist[] array by traversing over the arr[].
• Now, traverse on the dist[] array to find the count of consecutive elements.
• Simultaneously maintain the answer variable.

Below is the implementation of above approach:

C++

#include <bits/stdc++.h>

using namespace std;

int findLongestConseqSubseq( int arr[], int n)

{

int ans = 0, count = 0;

sort(arr, arr + n);

vector< int > v;

v.push_back(arr[0]);

for ( int i = 1; i < n; i++) {

if (arr[i] != arr[i - 1])

v.push_back(arr[i]);

}

for ( int i = 0; i < v.size(); i++) {

if (i > 0 && v[i] == v[i - 1] + 1)

count++;

else

count = 1;

ans = max(ans, count);

}

return ans;

}

int main()

{

int arr[] = { 1, 2, 2, 3 };

int n = sizeof arr / sizeof arr[0];

cout << "Length of the Longest contiguous subsequence "

"is "

<< findLongestConseqSubseq(arr, n);

return 0;

}

Java

import java.io.*;

import java.util.*;

class GFG {

static int findLongestConseqSubseq( int arr[], int n)

{

Arrays.sort(arr);

int ans = 0 , count = 0 ;

ArrayList<Integer> v = new ArrayList<Integer>();

v.add(arr[ 0 ]);

for ( int i = 1 ; i < n; i++) {

if (arr[i] != arr[i - 1 ])

v.add(arr[i]);

}

for ( int i = 0 ; i < v.size(); i++) {

if (i > 0 && v.get(i) == v.get(i - 1 ) + 1 )

count++;

else

count = 1 ;

ans = Math.max(ans, count);

}

return ans;

}

public static void main(String[] args)

{

int arr[] = { 1 , 9 , 3 , 10 , 4 , 20 , 2 };

int n = arr.length;

System.out.println(

"Length of the Longest "

+ "contiguous subsequence is "

+ findLongestConseqSubseq(arr, n));

}

}

Python3

def findLongestConseqSubseq(arr, n):

ans = 0

count = 0

arr.sort()

v = []

v.append(arr[ 0 ])

for i in range ( 1 , n):

if (arr[i] ! = arr[i - 1 ]):

v.append(arr[i])

for i in range ( len (v)):

if (i > 0 and v[i] = = v[i - 1 ] + 1 ):

count + = 1

else :

count = 1

ans = max (ans, count)

return ans

arr = [ 1 , 2 , 2 , 3 ]

n = len (arr)

print ( "Length of the Longest contiguous subsequence is" ,

findLongestConseqSubseq(arr, n))

C#

using System;

using System.Collections.Generic;

class GFG {

static int findLongestConseqSubseq( int [] arr, int n)

{

Array.Sort(arr);

int ans = 0, count = 0;

List< int > v = new List< int >();

v.Add(10);

for ( int i = 1; i < n; i++) {

if (arr[i] != arr[i - 1])

v.Add(arr[i]);

}

for ( int i = 0; i < v.Count; i++) {

if (i > 0 && v[i] == v[i - 1] + 1)

count++;

else

count = 1;

ans = Math.Max(ans, count);

}

return ans;

}

static void Main()

{

int [] arr = { 1, 9, 3, 10, 4, 20, 2 };

int n = arr.Length;

Console.WriteLine(

"Length of the Longest "

+ "contiguous subsequence is "

+ findLongestConseqSubseq(arr, n));

}

}

Javascript

<script>

function findLongestConseqSubseq(arr, n) {

let ans = 0, count = 0;

arr.sort( function (a, b) { return a - b; })

var v = [];

v.push(arr[0]);

for (let i = 1; i < n; i++) {

if (arr[i] != arr[i - 1])

v.push(arr[i]);

}

for (let i = 0; i < v.length; i++) {

if (i > 0 && v[i] == v[i - 1] + 1)

count++;

else

count = 1;

ans = Math.max(ans, count);

}

return ans;

}

let arr = [1, 2, 2, 3];

let n = arr.length;

document.write(

"Length of the Longest contiguous subsequence is "

+findLongestConseqSubseq(arr, n)

);

</script>

Output

Length of the Longest contiguous subsequence is 3

Time complexity: O(Nlog(N)), Time to sort the array is O(Nlog(N)).
Auxiliary space: O(N). Extra space is needed for storing distinct elements.

Longest Consecutive Subsequence using Hashing :

The idea is to use Hashing. We first insert all elements in a Set. Then check all the possible starts of consecutive subsequences.

Illustration:

Below image is the dry run for example arr[] = {1, 9, 3, 10, 4, 20, 2}:

Follow the steps below to solve the problem:

• Create an empty hash.
• Insert all array elements to hash.
• Do the following for every element arr[i]
• Check if this element is the starting point of a subsequence. To check this, simply look for arr[i] – 1 in the hash, if not found, then this is the first element of a subsequence.
• If this element is the first element, then count the number of elements in the consecutive starting with this element. Iterate from arr[i] + 1 till the last element that can be found.
• If the count is more than the previous longest subsequence found, then update this.

Below is the implementation of the above approach:

C++

#include <bits/stdc++.h>

using namespace std;

int findLongestConseqSubseq( int arr[], int n)

{

unordered_set< int > S;

int ans = 0;

for ( int i = 0; i < n; i++)

S.insert(arr[i]);

for ( int i = 0; i < n; i++) {

if (S.find(arr[i] - 1) == S.end()) {

int j = arr[i];

while (S.find(j) != S.end())

j++;

ans = max(ans, j - arr[i]);

}

}

return ans;

}

int main()

{

int arr[] = { 1, 9, 3, 10, 4, 20, 2 };

int n = sizeof arr / sizeof arr[0];

cout << "Length of the Longest contiguous subsequence "

"is "

<< findLongestConseqSubseq(arr, n);

return 0;

}

Java

import java.io.*;

import java.util.*;

class ArrayElements {

static int findLongestConseqSubseq( int arr[], int n)

{

HashSet<Integer> S = new HashSet<Integer>();

int ans = 0 ;

for ( int i = 0 ; i < n; ++i)

S.add(arr[i]);

for ( int i = 0 ; i < n; ++i) {

if (!S.contains(arr[i] - 1 )) {

int j = arr[i];

while (S.contains(j))

j++;

if (ans < j - arr[i])

ans = j - arr[i];

}

}

return ans;

}

public static void main(String args[])

{

int arr[] = { 1 , 9 , 3 , 10 , 4 , 20 , 2 };

int n = arr.length;

System.out.println(

"Length of the Longest consecutive subsequence is "

+ findLongestConseqSubseq(arr, n));

}

}

Python3

def findLongestConseqSubseq(arr, n):

s = set ()

ans = 0

for ele in arr:

s.add(ele)

for i in range (n):

if (arr[i] - 1 ) not in s:

j = arr[i]

while (j in s):

j + = 1

ans = max (ans, j - arr[i])

return ans

if __name__ = = '__main__' :

n = 7

arr = [ 1 , 9 , 3 , 10 , 4 , 20 , 2 ]

print ( "Length of the Longest contiguous subsequence is " ,

findLongestConseqSubseq(arr, n))

C#

using System;

using System.Collections.Generic;

public class ArrayElements {

public static int findLongestConseqSubseq( int [] arr,

int n)

{

HashSet< int > S = new HashSet< int >();

int ans = 0;

for ( int i = 0; i < n; ++i) {

S.Add(arr[i]);

}

for ( int i = 0; i < n; ++i) {

if (!S.Contains(arr[i] - 1)) {

int j = arr[i];

while (S.Contains(j)) {

j++;

}

if (ans < j - arr[i]) {

ans = j - arr[i];

}

}

}

return ans;

}

public static void Main( string [] args)

{

int [] arr = new int [] { 1, 9, 3, 10, 4, 20, 2 };

int n = arr.Length;

Console.WriteLine(

"Length of the Longest consecutive subsequence is "

+ findLongestConseqSubseq(arr, n));

}

}

Javascript

<script>

function findLongestConseqSubseq(arr, n) {

let S = new Set();

let ans = 0;

for (let i = 0; i < n; i++)

S.add(arr[i]);

for (let i = 0; i < n; i++)

{

if (!S.has(arr[i] - 1))

{

let j = arr[i];

while (S.has(j))

j++;

ans = Math.max(ans, j - arr[i]);

}

}

return ans;

}

let arr = [1, 9, 3, 10, 4, 20, 2];

let n = arr.length;

document.write( "Length of the Longest contiguous subsequence is "

+ findLongestConseqSubseq(arr, n));

</script>

Output

Length of the Longest contiguous subsequence is 4

Time complexity: O(N), Only one traversal is needed and the time complexity is O(n) under the assumption that hash insert and search takes O(1) time.
Auxiliary space: O(N), To store every element in the hashmap O(n) space is needed

Longest Consecutive Subsequence using Priority Queue :

The Idea is to use Priority Queue. Using priority queue it will sort the elements and eventually it will help to find consecutive elements.

Illustration:

Input: arr[] = {1, 9, 3, 10, 4, 20, 2}

Insert all the elements in the Priority Queue:

1

2

3

4

9

10

20

Initialise variable prev with first element of priority queue, prev will contain last element has been picked and it will help to check whether the current element is contributing for consecutive sequence or not.

prev = 1, countConsecutive = 1, ans = 1

Run the algorithm till the priority queue becomes empty.

2

3

4

9

10

20

• current element is 2
  • prev + 1 == 2, therefore increment countConsecutive by 1
  • countConsecutive = countConsecutive + 1 = 1 + 1 = 2
  • update prev with current element, prev = 2
  • pop the current element
  • ans = max(ans, countConsecutive) = (1, 2) = 2

3

4

9

10

20

• current element is 3
  • prev + 1 == 3, therefore increment countConsecutive by 1
  • countConsecutive = countConsecutive + 1 = 2 + 1 = 3
  • update prev with current element, prev = 3
  • pop the current element
  • ans = max(ans, countConsecutive) = (2, 3) = 3

4

9

10

20

• current element is 4
  • prev + 1 == 4, therefore increment countConsecutive by 1
  • countConsecutive = countConsecutive + 1 = 3 + 1 = 4
  • update prev with current element, prev = 4
  • pop the current element
  • ans = max(ans, countConsecutive) = (3, 4) = 4

9

10

20

• current element is 9
  • prev + 1 != 9, therefore re-initialise countConsecutive by 1
  • countConsecutive = 1
  • update prev with current element, prev = 9
  • pop the current element
  • ans = max(ans, countConsecutive) = (4, 1) = 4

10

20

• current element is 10
  • prev + 1 == 10, therefore increment countConsecutive by 1
  • countConsecutive = countConsecutive + 1 = 1 + 1 = 2
  • update prev with current element, prev = 10
  • pop the current element
  • ans = max(ans, countConsecutive) = (4, 2) =4

20

• current element is 20
  • prev + 1 != 20, therefore re-initialise countConsecutive by 1
  • countConsecutive = 1
  • update prev with current element, prev = 20
  • pop the current element
  • ans = max(ans, countConsecutive) = (4, 1) = 4

Hence, the longest consecutive subsequence is 4.

Follow the steps below to solve the problem:

• Create a Priority Queue to store the element
• Store the first element in a variable
• Remove it from the Priority Queue
• Check the difference between this removed first element and the new peek element
• If the difference is equal to 1 increase the count by 1 and repeats step 2 and step 3
• If the difference is greater than 1 set counter to 1 and repeat step 2 and step 3
• if the difference is equal to 0 repeat step 2 and 3
• if counter greater than the previous maximum then store counter to maximum
• Continue step 4 to 7 until we reach the end of the Priority Queue
• Return the maximum value

Below is the implementation of the above approach:

C++

#include <bits/stdc++.h>

using namespace std;

int findLongestConseqSubseq( int arr[], int N)

{

priority_queue< int , vector< int >, greater< int > > pq;

for ( int i = 0; i < N; i++) {

pq.push(arr[i]);

}

int prev = pq.top();

pq.pop();

int c = 1;

int max = 1;

while (!pq.empty()) {

if (pq.top() - prev > 1) {

c = 1;

prev = pq.top();

pq.pop();

}

else if (pq.top() - prev == 0) {

prev = pq.top();

pq.pop();

}

else {

c++;

prev = pq.top();

pq.pop();

}

if (max < c) {

max = c;

}

}

return max;

}

int main()

{

int arr[] = { 1, 9, 3, 10, 4, 20, 2 };

int n = 7;

cout << "Length of the Longest consecutive subsequence "

"is "

<< findLongestConseqSubseq(arr, n);

return 0;

}

Java

import java.io.*;

import java.util.PriorityQueue;

public class Longset_Sub {

static int findLongestConseqSubseq( int arr[], int N)

{

PriorityQueue<Integer> pq

= new PriorityQueue<Integer>();

for ( int i = 0 ; i < N; i++) {

pq.add(arr[i]);

}

int prev = pq.poll();

int c = 1 ;

int max = 1 ;

for ( int i = 1 ; i < N; i++) {

if (pq.peek() - prev > 1 ) {

c = 1 ;

prev = pq.poll();

}

else if (pq.peek() - prev == 0 ) {

prev = pq.poll();

}

else {

c++;

prev = pq.poll();

}

if (max < c) {

max = c;

}

}

return max;

}

public static void main(String args[])

throws IOException

{

int arr[] = { 1 , 9 , 3 , 10 , 4 , 20 , 2 };

int n = arr.length;

System.out.println(

"Length of the Longest consecutive subsequence is "

+ findLongestConseqSubseq(arr, n));

}

}

Python3

import bisect

def findLongestConseqSubseq(arr, N):

pq = []

for i in range (N):

bisect.insort(pq, arr[i])

prev = pq[ 0 ]

pq.pop( 0 )

c = 1

max = 1

while ( len (pq)):

if (pq[ 0 ] - prev > 1 ):

c = 1

prev = pq[ 0 ]

pq.pop( 0 )

elif (pq[ 0 ] - prev = = 0 ):

prev = pq[ 0 ]

pq.pop( 0 )

else :

c = c + 1

prev = pq[ 0 ]

pq.pop( 0 )

if ( max < c):

max = c

return max

arr = [ 1 , 9 , 3 , 10 , 4 , 20 , 2 ]

n = 7

print ( "Length of the Longest consecutive subsequence is {}" . format (

findLongestConseqSubseq(arr, n)))

C#

using System;

using System.Collections.Generic;

class GFG {

static int findLongestConseqSubseq( int [] arr, int N)

{

List< int > pq = new List< int >();

for ( int i = 0; i < N; i++) {

pq.Add(arr[i]);

pq.Sort();

}

int prev = pq[0];

int c = 1;

int max = 1;

for ( int i = 1; i < N; i++) {

if (pq[0] - prev > 1) {

c = 1;

prev = pq[0];

pq.RemoveAt(0);

}

else if (pq[0] - prev == 0) {

prev = pq[0];

pq.RemoveAt(0);

}

else {

c++;

prev = pq[0];

pq.RemoveAt(0);

}

if (max < c) {

max = c;

}

}

return max;

}

public static void Main()

{

int [] arr = { 1, 9, 3, 10, 4, 20, 2 };

int n = arr.Length;

Console.WriteLine(

"Length of the Longest consecutive subsequence is "

+ findLongestConseqSubseq(arr, n));

}

}

Output

Length of the Longest consecutive subsequence is 4

Time Complexity: O(N*log(N)), Time required to push and pop N elements is logN for each element.
Auxiliary Space: O(N), Space required by priority queue to store N elements.

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Source: https://www.geeksforgeeks.org/longest-consecutive-subsequence/

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