Sort an array after applying the given equation

Try it on GfG Practice

Given an integer array arr[] sorted in ascending order, along with three integers: A, B, and C. The task is to transform each element x in the array using the quadratic function A*(x^2) + B*x + C. After applying this transformation to every element, return the modified array in sorted order.

Examples:

Input: arr[] = [-4, -2, 0, 2, 4], A = 1, B = 3, C = 5
Output: [3, 5, 9, 15, 33]
Explanation: After applying f(x) = 1*x²+ 3*x + 5 to each x, we get [9, 3, 5, 15, 33]. After sorting this array, the array becomes [3, 5, 9, 15, 33].

Input: arr[] = [-3, -1, 2, 4], A = -1, B = 0, C = 0
Output: [-16, -9, -4, -1]
Explanation: After applying f(x) = -1*x² to each x, we get [-9, -1, -4, -16 ]. After sorting this array, the array becomes [-16, -9, -4, -1].

Input: arr[] = [-1, 0, 1, 2, 3, 4], A = -1, B = 2, C = -1
Output: [-9, -4, -4, -1, -1, 0]

[Brute Force Approach] Apply the Equation and Sort - O(n*log(n)) Time and O(1) Space

The idea is to directly transform each element in the array, i.e. for each element x in arr[], apply the given equation A*x² + B*x + C in-place and then simply sort the array in ascending order.

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

// Function to apply quadratic transformation
int evaluate(int x, int A, int B, int C) {
    return A * x * x + B * x + C;
}

// Function to transform and sort the array in-place
vector<int> sortArray(vector<int> &arr, int A, int B, int C) {
    int n = arr.size();
    
    vector<int> transformed;
    // Apply the transformation
    for (int i = 0; i < n; i++) {
        transformed.push_back(evaluate(arr[i], A, B, C));
    }

    // Sort the transformed array
    sort(transformed.begin(), transformed.end());

    return transformed;
}

int main() {
    vector<int> arr = {-4, -2, 0, 2, 4};
    int A = 1, B = 3, C = 5;

    vector<int> res = sortArray(arr, A, B, C);

    for (int val : res) {
        cout << val << " ";
    }

    return 0;
}

import java.util.*;

class GfG {

    // Function to apply quadratic transformation
    static int evaluate(int x, int A, int B, int C) {
        return A * x * x + B * x + C;
    }

    // Function to transform and sort the array (returns ArrayList<Integer>)
    static ArrayList<Integer> sortArray(int[] arr, int A, int B, int C) {
        int n = arr.length;
        Integer[] transformed = new Integer[n];

        // Apply the transformation and store in array
        for (int i = 0; i < n; i++) {
            transformed[i] = evaluate(arr[i], A, B, C);
        }

        // Sort the array
        Arrays.sort(transformed);

        // Convert to ArrayList and return
        return new ArrayList<>(Arrays.asList(transformed));
    }

    public static void main(String[] args) {
        int[] arr = {-4, -2, 0, 2, 4};
        int A = 1, B = 3, C = 5;

        ArrayList<Integer> res = sortArray(arr, A, B, C);

        for (int val : res) {
            System.out.print(val + " ");
        }
    }
}

# Python Code to Sort array after applying
# equation using Brute Force Approach

# Function to apply quadratic transformation
def evaluate(x, A, B, C):
    return A * x * x + B * x + C

# Function to transform and sort the array, returning a new list
def sortArray(arr, A, B, C):
    # Create a new array with the transformed values
    transformed = [evaluate(x, A, B, C) for x in arr]
    
    # Sort the transformed array
    transformed.sort()
    
    return transformed

if __name__ == "__main__":
    arr = [-4, -2, 0, 2, 4]
    A, B, C = 1, 3, 5

    res = sortArray(arr, A, B, C)

    for val in res:
        print(val, end=" ")

using System;
using System.Collections.Generic;

class GfG{
    // Function to apply quadratic transformation
    static int evaluate(int x, int A, int B, int C){
        
        return A * x * x + B * x + C;
    }

    // Function to transform and sort the array (returns List<int>)
    static List<int> sortArray(int[] arr, int A, int B, int C){
        
        int n = arr.Length;
        int[] transformed = new int[n];

        // Apply transformation
        for (int i = 0; i < n; i++){
            
            transformed[i] = evaluate(arr[i], A, B, C);
        }

        // Sort the transformed array
        Array.Sort(transformed);

        // Convert to List<int> and return
        return new List<int>(transformed);
    }

    static void Main(){
        
        int[] arr = { -4, -2, 0, 2, 4 };
        int A = 1, B = 3, C = 5;

        List<int> res = sortArray(arr, A, B, C);

        foreach (int val in res){
            
            Console.Write(val + " ");
        }
    }
}

// JavaScript Code to Sort array after applying
// equation using Brute Force Approach

// Function to apply quadratic transformation
function evaluate(x, A, B, C) {
    return A * x * x + B * x + C;
}

// Function to transform and sort the array in-place
function sortArray(arr, A, B, C) {
    let n = arr.length;
    
    let transformed = [];
    // Apply the transformation in-place
    for (let i = 0; i < n; i++) {
        transformed.push(evaluate(arr[i], A, B, C));
    }

    // Sort the transformed array
    transformed.sort((a, b) => a - b);

    return transformed;
}

// Driver Code
let arr = [-4, -2, 0, 2, 4];
let A = 1, B = 3, C = 5;

let res = sortArray(arr, A, B, C);

for (let val of res) {
    process.stdout.write(val + " ");
}

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[Expected Approach] Using Two Pointers - O(n) Time and O(n) Space

The idea is to use the fact that input array is sorted and apply a Two-Pointer approach. After applying the quadratic transformation to each element, the smallest and largest values will always be at the ends of the array (Why? The equation given is parabolic. So the result of applying it to a sorted array will result in an array that will have a maximum/minimum with the sub-arrays to its left and right sorted)

We process from both ends using two pointers (left and right) moving towards each other. By comparing the transformed values of arr[left] and arr[right], we can fill a new array (newArr) starting either from the front or the back depending on whether the coefficient A is positive or negative. The observation here is:

• if A >= 0, the largest values will be at the end
• while if A < 0, the largest values will be at the beginning.

There are two distinct cases when working with a quadratic transformation of a sorted array, and the nature of the function depends on the coefficient A.

Case 1: A > 0 – Convex parabola (opens upwards) → Has a valley (minimum point)

• The largest transformed values lie at the ends of the array.
• We fill the result array from end to start using two pointers.

Case 2: A < 0 – Concave parabola (opens downwards) → Has a peak (maximum point)

• The smallest transformed values lie at the ends.
• We fill the result array from start to end using two pointers.

Step-by-Step Implementation:

• Start by initializing two pointers: left at the beginning and right at the end of the array.
• Create a new array newArr of the same size to store the transformed and sorted values.
• Determine the index where you will start filling newArr based on whether A is positive or negative.
• Loop through the array using the left and right pointers, comparing the transformed values of arr[left] and arr[right].
• Depending on the sign of A, place the larger of the two values at the appropriate position in newArr.
• After processing all elements, copy the sorted values from newArr back to the original arr.
• Finally, return the modified arr containing the transformed and sorted values.

Illustration:

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

// Function to apply quadratic transformation
int evaluate(int x, int A, int B, int C) {
    return A * x * x + B * x + C;
}

// Function to transform and sort the array 
vector<int> sortArray(vector<int> &arr, int A, int B, int C) {
    int n = arr.size();
    vector<int> newArr(n);

    int left = 0, right = n - 1;
    int index = (A >= 0) ? n - 1 : 0;

    // Two-pointer approach to fill newArr 
    while (left <= right) {
        int leftVal = evaluate(arr[left], A, B, C);
        int rightVal = evaluate(arr[right], A, B, C);

        if (A >= 0) {
            if (leftVal > rightVal) {
                newArr[index--] = leftVal;
                left++;
            } else {
                newArr[index--] = rightVal;
                right--;
            }
        } else {
            if (leftVal < rightVal) {
                newArr[index++] = leftVal;
                left++;
            } else {
                newArr[index++] = rightVal;
                right--;
            }
        }
    }

    return newArr;
}

int main() {
    vector<int> arr = {-4, -2, 0, 2, 4};
    int A = 1, B = 3, C = 5;

    vector<int> res = sortArray(arr, A, B, C);

    for (int val : res) {
        cout << val << " ";
    }

    return 0;
}

import java.util.*;

class GfG {

    // Function to apply quadratic transformation
    public static int evaluate(int x, int A, int B, int C) {
        return A * x * x + B * x + C;
    }

    // Function to transform and sort the array
    public static ArrayList<Integer> sortArray(int[] arr, int A, int B, int C) {
        int n = arr.length;
        int[] newArr = new int[n];

        int left = 0, right = n - 1;
        int index = (A >= 0) ? n - 1 : 0;

        // Two-pointer approach to fill newArr from end or start
        while (left <= right) {
            int leftVal = evaluate(arr[left], A, B, C);
            int rightVal = evaluate(arr[right], A, B, C);

            if (A >= 0) {
                if (leftVal > rightVal) {
                    newArr[index--] = leftVal;
                    left++;
                } else {
                    newArr[index--] = rightVal;
                    right--;
                }
            } else {
                if (leftVal < rightVal) {
                    newArr[index++] = leftVal;
                    left++;
                } else {
                    newArr[index++] = rightVal;
                    right--;
                }
            }
        }

        // Convert array to ArrayList and return
        ArrayList<Integer> result = new ArrayList<>();
        for (int val : newArr) {
            result.add(val);
        }

        return result;
    }

    public static void main(String[] args) {
        int[] arr = {-4, -2, 0, 2, 4};
        int A = 1, B = 3, C = 5;

        ArrayList<Integer> res = sortArray(arr, A, B, C);

        for (int val : res) {
            System.out.print(val + " ");
        }
    }
}

# Python Code to Sort array after applying
# equation using Two Pointer Approach

# Function to apply quadratic transformation
def evaluate(x, A, B, C):
    return A * x * x + B * x + C

# Function to transform and sort the array
def sortArray(arr, A, B, C):
    n = len(arr)
    newArr = [0] * n

    left, right = 0, n - 1
    index = n - 1 if A >= 0 else 0

    # Two-pointer approach to fill newArr
    # from end or start
    while left <= right:
        leftVal = evaluate(arr[left], A, B, C)
        rightVal = evaluate(arr[right], A, B, C)

        if A >= 0:
            # Fill from end
            if leftVal > rightVal:
                newArr[index] = leftVal
                left += 1
                index -= 1
            else:
                newArr[index] = rightVal
                right -= 1
                index -= 1
        else:
            # Fill from start
            if leftVal < rightVal:
                newArr[index] = leftVal
                left += 1
                index += 1
            else:
                newArr[index] = rightVal
                right -= 1
                index += 1

    return newArr


if __name__ == "__main__":
    arr = [-4, -2, 0, 2, 4]
    A = 1
    B = 3
    C = 5

    res = sortArray(arr, A, B, C)
    print(*res)

using System;
using System.Collections.Generic;

class GfG {

    // Function to apply quadratic transformation
    public static int evaluate(int x, int A, int B, int C) {
        return A * x * x + B * x + C;
    }

    // Function to transform and sort the array
    public static List<int> sortArray(int[] arr, int A, int B, int C) {
        int n = arr.Length;
        int[] newArr = new int[n];

        int left = 0, right = n - 1;
        int index = (A >= 0) ? n - 1 : 0;

        // Two-pointer approach to fill newArr
        while (left <= right) {
            int leftVal = evaluate(arr[left], A, B, C);
            int rightVal = evaluate(arr[right], A, B, C);

            if (A >= 0) {
                if (leftVal > rightVal) {
                    newArr[index--] = leftVal;
                    left++;
                } else {
                    newArr[index--] = rightVal;
                    right--;
                }
            } else {
                if (leftVal < rightVal) {
                    newArr[index++] = leftVal;
                    left++;
                } else {
                    newArr[index++] = rightVal;
                    right--;
                }
            }
        }

        return new List<int>(newArr);
    }

    public static void Main(string[] args) {
        int[] arr = {-4, -2, 0, 2, 4};
        int A = 1, B = 3, C = 5;

        List<int> res = sortArray(arr, A, B, C);

        foreach (int val in res) {
            Console.Write(val + " ");
        }
    }
}

// JavaScript Code to Sort array after applying
// equation using Two Pointer Approach

// Function to apply quadratic transformation
function evaluate(x, A, B, C) {
    return A * x * x + B * x + C;
}

// Function to transform and sort the array
function sortArray(arr, A, B, C) {
    let n = arr.length;
    let newArr = new Array(n);

    let left = 0, right = n - 1;
    let index = (A >= 0) ? n - 1 : 0;

    // Two-pointer approach to fill newArr
    // from end or start
    while (left <= right) {
        let leftVal = evaluate(arr[left], A, B, C);
        let rightVal = evaluate(arr[right], A, B, C);

        if (A >= 0) {
            // Fill from end
            if (leftVal > rightVal) {
                newArr[index] = leftVal;
                left++;
                index--;
            } else {
                newArr[index] = rightVal;
                right--;
                index--;
            }
        } else {
            // Fill from start
            if (leftVal < rightVal) {
                newArr[index] = leftVal;
                left++;
                index++;
            } else {
                newArr[index] = rightVal;
                right--;
                index++;
            }
        }
    }

    // Assign values back to arr
    for (let i = 0; i < n; i++) {
        arr[i] = newArr[i];
    }

    return arr;
}

// Driver Code
let arr = [-4, -2, 0, 2, 4];
let A = 1, B = 3, C = 5;

let res = sortArray(arr, A, B, C);

console.log(res.join(' '));

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