Maximum Frequency by replacing with elements in a given range

Given a 0-indexed array arr[] of N positive integers and two integers K and X. Find the maximum frequency of any element(not necessary to be present in the array) you can make after performing the below operation at most K times-

• Choose an index i and replace arr[i] with any integer from the range [arr[i] – X , arr[i] + X]

Note: You are allowed to operate on each index at most once.

Examples:

Input: N = 4, K = 2, X = 2, arr[] = {1, 2, 3, 4}
Output: 3
Explanation: After performing the operation on index 1 and 2. Modified array is {1, 1, 1, 4} so the maximum frequency of 1 is 3. It can be proven that we can’t make the frequency of any element greater than 3.

Input: N = 5, K = 3, X = 2, arr[] = {1, 3, 8, 12, 18}
Output: 2
Explanation: After performing the operation on index 0. Modified array is {3, 3, 8, 12, 18}, So the maximum frequency that can be made is 2.

Approach: To solve the problem, follow the below idea:

The main idea is to calculate frequency of each element and iterate through each unique element in the array, considering the range of possible values after the operation mentioned i.e. [arr[i] – X , arr[i] + X], and calculating the maximum frequency achievable for each element using lower bound and upper bound. The result is the maximum frequency among all elements.

Step-by-step algorithm:

• Create a frequency map (dit) to store the count of each element in the array.
• Create sets (s and reach) to store unique elements and their corresponding reachable values after the operation.
• Combine the sets to form a set of all potential values (s).
• Create a sorted array (sortedArr) from the original array.
• Iterate through each unique element in the set s.
  • Calculate the upper and lower bounds in the sorted array for the range of values after the operation.
  • Determine the number of operations that can be performed for the given element (operations).
  • Update the maximum frequency achievable for the current element.
• Return the maximum frequency among all elements.

Below is the implementation of the algorithm:

C++

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

// Function to find index of upper bound element
int upperBound(const vector<int>& arr, int target)
{
    return upper_bound(arr.begin(), arr.end(), target)
        - arr.begin();
}

// Function to find index of lower bound element
int lowerBound(const vector<int>& arr, int target)
{
    return lower_bound(arr.begin(), arr.end(), target)
        - arr.begin();
}

// Function to find element with maximum Frequency
int MaxFrequency(int N, vector<int>& arr, int K, int X)
{

    // Store the elements frequency in map
    unordered_map<int, int> dit;
    for (int t : arr) {
        dit[t]++;
    }

    // Store the addition in set
    unordered_set<int> s, reach;
    for (int e : arr) {
        s.insert(e);
        reach.insert(e + X);
    }

    s.insert(reach.begin(), reach.end());

    vector<int> sortedArr(arr.begin(), arr.end());
    sort(sortedArr.begin(), sortedArr.end());

    int ans = 1;

    // For each entry in set find the index of
    // lower bound and upper bound
    for (int e : s) {
        int upper = upperBound(sortedArr, e + X);
        int lower = lowerBound(sortedArr, e - X);
        int operations = min(upper - lower - dit[e], K);

        // Update the maximum frequency element
        ans = max(ans, operations + dit[e]);
    }

    // Return maxx
    return ans;
}

// Driver code
int main()
{

    int n = 4;
    int k = 2;
    int x = 2;

    vector<int> arr = { 1, 2, 3, 4 };

    // Function call
    cout << MaxFrequency(n, arr, k, x);
    return 0;
}

Java

import java.util.*;

public class MaxFrequency {

    // Function to find index of upper bound element
    static int upperBound(List<Integer> arr, int target) {
        return Collections.binarySearch(arr, target, Comparator.naturalOrder());
    }

    // Function to find index of lower bound element
    static int lowerBound(List<Integer> arr, int target) {
        return Collections.binarySearch(arr, target, Comparator.naturalOrder());
    }

    // Function to find element with maximum Frequency
    static int maxFrequency(int N, List<Integer> arr, int K, int X) {

        // Store the elements frequency in map
        Map<Integer, Integer> dit = new HashMap<>();
        for (int t : arr) {
            dit.put(t, dit.getOrDefault(t, 0) + 1);
        }

        // Store the addition in set
        Set<Integer> s = new HashSet<>();
        Set<Integer> reach = new HashSet<>();
        for (int e : arr) {
            s.add(e);
            reach.add(e + X);
        }

        s.addAll(reach);

        List<Integer> sortedArr = new ArrayList<>(arr);
        Collections.sort(sortedArr);

        int ans = 1;

        // For each entry in set find the index of
        // lower bound and upper bound
        for (int e : s) {
            int upper = upperBound(sortedArr, e + X);
            int lower = lowerBound(sortedArr, e - X);
            int operations = Math.min(upper - lower - dit.getOrDefault(e, 0), K);

            // Update the maximum frequency element
            ans = Math.max(ans, operations + dit.getOrDefault(e, 0));
        }

        // Return max
        return ans;
    }

    // Driver code
    public static void main(String[] args) {

        int n = 4;
        int k = 2;
        int x = 2;

        List<Integer> arr = Arrays.asList(1, 2, 3, 4);

        // Function call
        System.out.println(maxFrequency(n, arr, k, x));
    }
}

// This code is contributed by shivamgupta310570

C#

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    // Function to find index of upper bound element
    static int UpperBound(List<int> arr, int target)
    {
        return arr.BinarySearch(target);
    }

    // Function to find index of lower bound element
    static int LowerBound(List<int> arr, int target)
    {
        return arr.BinarySearch(target);
    }

    // Function to find element with maximum Frequency
    static int MaxFrequency(int N, List<int> arr, int K, int X)
    {
        // Store the elements frequency in Dictionary
        Dictionary<int, int> dict = new Dictionary<int, int>();
        foreach (int t in arr)
        {
            if (dict.ContainsKey(t))
                dict[t]++;
            else
                dict[t] = 1;
        }

        // Store the addition in HashSet
        HashSet<int> s = new HashSet<int>();
        HashSet<int> reach = new HashSet<int>();
        foreach (int e in arr)
        {
            s.Add(e);
            reach.Add(e + X);
        }

        s.UnionWith(reach);

        List<int> sortedArr = new List<int>(arr);
        sortedArr.Sort();

        int ans = 1;

        // For each entry in set find the index of
        // lower bound and upper bound
        foreach (int e in s)
        {
            int upper = UpperBound(sortedArr, e + X);
            int lower = LowerBound(sortedArr, e - X);
            int operations = Math.Min(upper - lower - (dict.ContainsKey(e) ? dict[e] : 0), K);

            // Update the maximum frequency element
            ans = Math.Max(ans, operations + (dict.ContainsKey(e) ? dict[e] : 0));
        }

        // Return max
        return ans;
    }

    // Driver code
    static void Main(string[] args)
    {
        int n = 4;
        int k = 2;
        int x = 2;

        List<int> arr = new List<int> { 1, 2, 3, 4 };

        // Function call
        Console.WriteLine(MaxFrequency(n, arr, k, x));
    }
}

Javascript

// JavaScript Implementation

// Function to find index of upper bound element
function upperBound(arr, target) {
    let i = 0;
    let length = arr.length;
    while (i < length) {
        let mid = i + Math.floor((length - i) / 2);
        if (arr[mid] <= target) {
            i = mid + 1;
        } else {
            length = mid;
        }
    }
    return i;
}

// Function to find index of lower bound element
function lowerBound(arr, target) {
    let i = 0;
    let length = arr.length;
    while (i < length) {
        let mid = i + Math.floor((length - i) / 2);
        if (arr[mid] < target) {
            i = mid + 1;
        } else {
            length = mid;
        }
    }
    return i;
}

// Function to find element with maximum Frequency
function maxFrequency(N, arr, K, X) {
    // Store the elements frequency in map
    let dit = new Map();
    for (let t of arr) {
        dit.set(t, (dit.get(t) || 0) + 1);
    }

    // Store the addition in set
    let s = new Set();
    let reach = new Set();
    for (let e of arr) {
        s.add(e);
        reach.add(e + X);
    }

    s = new Set([...s, ...reach]);

    let sortedArr = arr.slice().sort((a, b) => a - b);

    let ans = 1;

    // For each entry in set find the index of
    // lower bound and upper bound
    for (let e of s) {
        let upper = upperBound(sortedArr, e + X);
        let lower = lowerBound(sortedArr, e - X);
        let operations = Math.min(upper - lower - (dit.get(e) || 0), K);

        // Update the maximum frequency element
        ans = Math.max(ans, operations + (dit.get(e) || 0));
    }

    // Return max
    return ans;
}

// Driver code
let n = 4;
let k = 2;
let x = 2;

let arr = [1, 2, 3, 4];

// Function call
console.log(maxFrequency(n, arr, k, x));

Python3

# Python Implementation
from typing import List
from collections import defaultdict

# Function to find index of upper bound element
def upper_bound(arr: List[int], target: int) -> int:
    i = 0
    length = len(arr)
    while i < length:
        mid = i + (length - i) // 2
        if arr[mid] <= target:
            i = mid + 1
        else:
            length = mid
    return i

# Function to find index of lower bound element
def lower_bound(arr: List[int], target: int) -> int:
    i = 0
    length = len(arr)
    while i < length:
        mid = i + (length - i) // 2
        if arr[mid] < target:
            i = mid + 1
        else:
            length = mid
    return i

# Function to find element with maximum Frequency
def max_frequency(N: int, arr: List[int], K: int, X: int) -> int:
    # Store the elements frequency in map
    dit = defaultdict(int)
    for t in arr:
        dit[t] += 1

    # Store the addition in set
    s = set()
    reach = set()
    for e in arr:
        s.add(e)
        reach.add(e + X)

    s = s.union(reach)

    sorted_arr = sorted(arr)

    ans = 1

    # For each entry in set find the index of
    # lower bound and upper bound
    for e in s:
        upper = upper_bound(sorted_arr, e + X)
        lower = lower_bound(sorted_arr, e - X)
        operations = min(upper - lower - dit[e], K)

        # Update the maximum frequency element
        ans = max(ans, operations + dit[e])

    # Return max
    return ans

# Driver code
if __name__ == "__main__":
    n = 4
    k = 2
    x = 2

    arr = [1, 2, 3, 4]

    # Function call
    print(max_frequency(n, arr, k, x))

Output

3

Time Complexity: O(N log N), where N is the size of input array arr[].
Auxiliary Space: O(N)