Longest subarray whose elements can be made equal by maximum K increments

Given an array arr[] of positive integers of size N and a positive integer K, the task is to find the maximum possible length of subarray which can be made equal by adding some integer value to each element of the sub-array such that the sum of the added elements does not exceed K.

Examples:

Input: N = 5, arr[] = {1, 4, 9, 3, 6}, K = 9
Output: 3
Explanation:
{1, 4} : {1+3, 4} = {4, 4}
{4, 9} : {4+5, 9} = {9, 9}
{3, 6} : {3+3, 6} = {6, 6}
{9, 3, 6} : {9, 3+6, 6+3} = {9, 9, 9}
Hence, the maximum length of such a subarray is 3.

Input: N = 6, arr[] = {2, 4, 7, 3, 8, 5}, K = 10
Output: 4

Approach: This problem can be solved by using dynamic programming.



  • Initialize:
    • dp[]: Stores the sum of elements that are added to the subarray.
    • deque: Stores the indices of the maximum element for each subarray.
    • pos: Index of the current position of the subarray.
    • ans: Length of maximum sub array.
    • mx: Maximum element of a sub array
    • pre: Previous index of the current subarray.
  • Traverse the array and check if deque is empty or not. If yes, then update the maximum element and the index of maximum element along with the indices of pre and pos.
  • Check if the currently added element is greater then K. If yes, then remove it from dp[] array and update the indices of pos and pre.
  • Finally update the maximum length of the valid sub array.

Below is the implementation of above approach:

C++

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// C++ code for the above approach
  
#include <bits/stdc++.h>
using namespace std;
  
// Function to find maximum
// possible length of subarray
int validSubArrLength(int arr[],
                      int N, int K)
{
    // Stores the sum of elements
    // that needs to be added to
    // the sub array
    int dp[N + 1];
  
    // Stores the index of the
    // current position of subarray
    int pos = 0;
  
    // Stores the maximum
    // length of subarray.
    int ans = 0;
  
    // Maximum element from
    // each subarray length
    int mx = 0;
  
    // Previous index of the
    // current subarray of
    // maximum length
    int pre = 0;
  
    // Deque to store the indices
    // of maximum element of
    // each sub array
    deque<int> q;
  
    // For each array element,
    // find the maximum length of
    // required subarray
    for (int i = 0; i < N; i++) {
        // Traverse the deque and
        // update the index of
        // maximum element.
        while (!q.empty()
               && arr[q.back()] < arr[i])
  
            q.pop_back();
  
        q.push_back(i);
  
        // If it is first element
        // then update maximum
        // and dp[]
        if (i == 0) {
            mx = arr[i];
            dp[i] = arr[i];
        }
  
        // Else check if current
        // element exceeds max
        else if (mx <= arr[i]) {
            // Update max and dp[]
            dp[i] = dp[i - 1] + arr[i];
            mx = arr[i];
        }
  
        else {
            dp[i] = dp[i - 1] + arr[i];
        }
  
        // Update the index of the
        // current maximum length
        // subarray
        if (pre == 0)
            pos = 0;
        else
            pos = pre - 1;
  
        // While current element
        // being added to dp[] array
        // exceeds K
        while ((i - pre + 1) * mx
                       - (dp[i] - dp[pos])
                   > K
               && pre < i) {
  
            // Update index of
            // current position and
            // the previous position
            pos = pre;
            pre++;
  
            // Remove elements
            // from deque and
            // update the
            // maximum element
            while (!q.empty()
                   && q.front() < pre
                   && pre < i) {
  
                q.pop_front();
                mx = arr[q.front()];
            }
        }
  
        // Update the maximum length
        // of the required subarray.
        ans = max(ans, i - pre + 1);
    }
    return ans;
}
  
// Driver Program
int main()
{
    int N = 6;
    int K = 8;
    int arr[] = { 2, 7, 1, 3, 4, 5 };
    cout << validSubArrLength(arr, N, K);
    return 0;
}

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Java

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// Java code for the above approach
import java.util.*;
  
class GFG{
  
// Function to find maximum
// possible length of subarray
static int validSubArrLength(int arr[],
                             int N, int K)
{
    // Stores the sum of elements
    // that needs to be added to
    // the sub array
    int []dp = new int[N + 1];
  
    // Stores the index of the
    // current position of subarray
    int pos = 0;
  
    // Stores the maximum
    // length of subarray.
    int ans = 0;
  
    // Maximum element from
    // each subarray length
    int mx = 0;
  
    // Previous index of the
    // current subarray of
    // maximum length
    int pre = 0;
  
    // Deque to store the indices
    // of maximum element of
    // each sub array
    Deque<Integer> q = new LinkedList<>();
  
    // For each array element,
    // find the maximum length of
    // required subarray
    for(int i = 0; i < N; i++) 
    {
          
       // Traverse the deque and
       // update the index of
       // maximum element.
       while (!q.isEmpty() && 
               arr[q.getLast()] < arr[i])
           q.removeLast();
       q.add(i);
         
       // If it is first element
       // then update maximum
       // and dp[]
       if (i == 0)
       {
           mx = arr[i];
           dp[i] = arr[i];
       }
         
       // Else check if current
       // element exceeds max
       else if (mx <= arr[i])
       {
             
           // Update max and dp[]
           dp[i] = dp[i - 1] + arr[i];
           mx = arr[i];
       }
       else
       {
           dp[i] = dp[i - 1] + arr[i];
       }
         
       // Update the index of the
       // current maximum length
       // subarray
       if (pre == 0)
           pos = 0;
       else
           pos = pre - 1;
         
       // While current element
       // being added to dp[] array
       // exceeds K
       while ((i - pre + 1) * mx - 
          (dp[i] - dp[pos]) > K && pre < i)
       {
             
           // Update index of
           // current position and
           // the previous position
           pos = pre;
           pre++;
             
           // Remove elements from 
           // deque and update the
           // maximum element
           while (!q.isEmpty() && 
                   q.peek() < pre && pre < i)
           {
               q.removeFirst();
               mx = arr[q.peek()];
           }
       }
         
       // Update the maximum length
       // of the required subarray.
       ans = Math.max(ans, i - pre + 1);
    }
    return ans;
}
  
// Driver code
public static void main(String[] args)
{
    int N = 6;
    int K = 8;
    int arr[] = { 2, 7, 1, 3, 4, 5 };
      
    System.out.print(validSubArrLength(arr, N, K));
}
}
  
// This code is contributed by amal kumar choubey

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Output:

4


Time Complexity: O(N2)
Auxillary Space Complexity: O(N)

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Improved By : Amal Kumar Choubey