Length of longest non-decreasing subsequence such that difference between adjacent elements is at most one

Last Updated : 23 Jul, 2025

Given an array arr[] consisting of N integers, the task is to find the length of the longest non-decreasing subsequence such that the difference between adjacent elements is at most 1.

Examples:

Input: arr[] = {8, 5, 4, 8, 4}
Output: 3
Explanation: {4, 4, 5}, {8, 8} are the two such non-decreasing subsequences of length 2 and 3 respectively. Therefore, the length of the longest of the two subsequences is 3.

Input: arr[] = {4, 13, 2, 3}
Output: 3
Explanation: {2, 3, 4}, {13} are the two such non-decreasing subsequences of length 3 and 1 respectively. Therefore, the length of the longest of the two subsequences is 3.

Approach: Follow the steps below to solve the problem:

Sort the array arr[] in increasing order.
Initialize a variable, say maxLen = 1, to store the maximum possible length of a subsequence. Initialize another variable, say len = 1 to store the current length for each subsequence.
Traverse the array arr[] with a pointer i and for each element:
- Check if abs(arr[i] - arr[i - 1]) ? 1. If found to be true, then increment len by 1. Update maxLen = max(maxLen, len).
- Otherwise, set len = 1 i.e. start a new subsequence.
Print the value of maxLen as the final answer.

Below is the implementation of the above approach:

C++

// C++ program to implement
// the above approach

#include <bits/stdc++.h>
using namespace std;

// Function to find the longest non-decreasing
// subsequence with difference between
// adjacent elements exactly equal to 1
void longestSequence(int arr[], int N)
{
  // Base case
  if (N == 0) {
    cout << 0;
    return;
  }

  // Sort the array in ascending order
  sort(arr, arr+N);

  // Stores the maximum length
  int maxLen = 1;

  int len = 1;

  // Traverse the array
  for (int i = 1; i < N; i++) {

    // If difference between current
    // pair of adjacent elements is 1 or 0
    if (arr[i] == arr[i - 1]
        || arr[i] == arr[i - 1] + 1) {
      len++;

      // Extend the current sequence
      // Update len and max_len
      maxLen = max(maxLen, len);
    }
    else {

      // Otherwise, start a new subsequence
      len = 1;
    }
  }

  // Print the maximum length
  cout << maxLen;
}


// Driver Code
int main()
{

  // Given array
  int arr[] = { 8, 5, 4, 8, 4 };

  // Size of the array
  int N = sizeof(arr) / sizeof(arr[0]);

  // Function call to find the longest
  // subsequence
  longestSequence(arr, N);

  return 0;
}

// This code is contributed by code_hunt.

Java

// Java program for the above approach

import java.io.*;
import java.util.*;
class GFG {

    // Function to find the longest non-decreasing
    // subsequence with difference between
    // adjacent elements exactly equal to 1
    static void longestSequence(int arr[], int N)
    {
        // Base case
        if (N == 0) {
            System.out.println(0);
            return;
        }

        // Sort the array in ascending order
        Arrays.sort(arr);

        // Stores the maximum length
        int maxLen = 1;

        int len = 1;

        // Traverse the array
        for (int i = 1; i < N; i++) {

            // If difference between current
            // pair of adjacent elements is 1 or 0
            if (arr[i] == arr[i - 1]
                || arr[i] == arr[i - 1] + 1) {
                len++;

                // Extend the current sequence
                // Update len and max_len
                maxLen = Math.max(maxLen, len);
            }
            else {

                // Otherwise, start a new subsequence
                len = 1;
            }
        }

        // Print the maximum length
        System.out.println(maxLen);
    }

    // Driver Code
    public static void main(String[] args)
    {
        // Given array
        int arr[] = { 8, 5, 4, 8, 4 };

        // Size of the array
        int N = arr.length;

        // Function call to find the longest
        // subsequence
        longestSequence(arr, N);
    }
}

Python3

# Python program for the above approach

# Function to find the longest non-decreasing
# subsequence with difference between
# adjacent elements exactly equal to 1
def longestSequence(arr, N):
  
    # Base case
    if (N == 0):
        print(0);
        return;

    # Sort the array in ascending order
    arr.sort();

    # Stores the maximum length
    maxLen = 1;
    len = 1;

    # Traverse the array
    for i in range(1,N):

        # If difference between current
        # pair of adjacent elements is 1 or 0
        if (arr[i] == arr[i - 1] or arr[i] == arr[i - 1] + 1):
            len += 1;

            # Extend the current sequence
            # Update len and max_len
            maxLen = max(maxLen, len);
        else:

            # Otherwise, start a new subsequence
            len = 1;

    # Print the maximum length
    print(maxLen);

# Driver Code
if __name__ == '__main__':
  
    # Given array
    arr = [8, 5, 4, 8, 4];

    # Size of the array
    N = len(arr);

    # Function call to find the longest
    # subsequence
    longestSequence(arr, N);

    # This code is contributed by 29AjayKumar

// C# program for the above approach
using System;
public class GFG 
{

  // Function to find the longest non-decreasing
  // subsequence with difference between
  // adjacent elements exactly equal to 1
  static void longestSequence(int []arr, int N)
  {

    // Base case
    if (N == 0) 
    {
      Console.WriteLine(0);
      return;
    }

    // Sort the array in ascending order
    Array.Sort(arr);

    // Stores the maximum length
    int maxLen = 1;
    int len = 1;

    // Traverse the array
    for (int i = 1; i < N; i++) {

      // If difference between current
      // pair of adjacent elements is 1 or 0
      if (arr[i] == arr[i - 1]
          || arr[i] == arr[i - 1] + 1) {
        len++;

        // Extend the current sequence
        // Update len and max_len
        maxLen = Math.Max(maxLen, len);
      }
      else {

        // Otherwise, start a new subsequence
        len = 1;
      }
    }

    // Print the maximum length
    Console.WriteLine(maxLen);
  }

  // Driver Code
  public static void Main(string[] args)
  {

    // Given array
    int []arr = { 8, 5, 4, 8, 4 };

    // Size of the array
    int N = arr.Length;

    // Function call to find the longest
    // subsequence
    longestSequence(arr, N);
  }
}

// This code is contributed by AnkThon

JavaScript

<script>

// Javascript program to implement
// the above approach

// Function to find the longest non-decreasing
// subsequence with difference between
// adjacent elements exactly equal to 1
function longestSequence(arr, N)
{
  // Base case
  if (N == 0) {
    document.write(0);
    return;
  }

  // Sort the array in ascending order
  arr.sort();

  // Stores the maximum length
  var maxLen = 1;

  var len = 1;
   var i;
  // Traverse the array
  for (i = 1; i < N; i++) {

    // If difference between current
    // pair of adjacent elements is 1 or 0
    if (arr[i] == arr[i - 1]
        || arr[i] == arr[i - 1] + 1) {
      len++;

      // Extend the current sequence
      // Update len and max_len
      maxLen = Math.max(maxLen, len);
    }
    else {

      // Otherwise, start a new subsequence
      len = 1;
    }
  }

  // Print the maximum length
  document.write(maxLen);
}


// Driver Code
  // Given array
  var arr = [8, 5, 4, 8, 4];

  // Size of the array
  var N = arr.length;

  // Function call to find the longest
  // subsequence
  longestSequence(arr, N);

</script>

Output:

Time Complexity: O(N * logN)
Auxiliary Space: O(1)

Longest non-decreasing subsequence having difference between adjacent elements less than D

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Length of longest non-decreasing subsequence such that difference between adjacent elements is at most one

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