Find initial sequence that produces a given Array by cyclic increments upto index P

Given an array arr[] consisting of N elements and an integer P, the task is to find the initial array from which given arr[] is produced by the following operations: 
 

  • An element arr[i] from the initial array is selected. The ith index is reduced to 0.
  • arr[i] indices are increased by 1 in a cyclic manner such that the last index to be incremented is P.

Examples: 
 

Input: arr[] = {4, 3, 1, 6}, P = 4 
Output: 3 2 5 4 
Explanation: 
The element arr[2] is chosen from the initial array. The following arr[i] operations modifies the given array in the following sequence: 
{3, 2, 0, 4} -> {3, 2, 0, 5} -> {4, 2, 0, 5} -> {4, 3, 0, 5} -> {4, 3, 1, 5} -> {4, 3, 1, 6}
Input: arr[] = {3, 2, 0, 2, 7}, P = 2 
Output: 2 1 4 1 6 
 

 

Approach: The above problem can be solved using below steps: 
 



  1. Find the minimum element in the array and subtract min – 1 from every index.
  2. Now start subtracting 1 from the (P – 1)th index and repeat for all indices on the left in a cyclic manner until an index becomes 0.
  3. Add the number of operations to that index.
  4. The current state of arr[] gives the required initial state. Print the array.

Below is the implementation of the above approach :
 

C++

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// C++ program to implement the
// above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to generate and return the
// required initial arrangement
void findArray(int* a,
               int n,
               int P)
{
    // Store the minimum element
    // in the array
    int mi = *min_element(a, a + n);
 
    // Store the number of increments
    int ctr = 0;
    mi = max(0, mi - 1);
 
    // Subtract mi - 1 from every index
    for (int i = 0; i < n; i++) {
 
        a[i] -= mi;
 
        ctr += mi;
    }
 
    // Start from the last index which
    // had been incremented
    int i = P - 1;
 
    // Stores the index chosen to
    // distribute its element
    int start = -1;
 
    // Traverse the array cyclically and
    // find the index whose element was
    // distributed
    while (1) {
 
        // If any index has its
        // value reduced to 0
        if (a[i] == 0) {
 
            // Index whose element was
            // distributed
            start = i;
 
            break;
        }
 
        a[i] -= 1;
        ctr += 1;
        i = (i - 1 + n) % n;
    }
 
    // Store the number of increments
    // at the starting index
    a[start] = ctr;
 
    // Print the original array
    for (int i = 0; i < n; i++) {
        cout << a[i] << ", ";
    }
}
 
// Driver Code
int main()
{
    int N = 5;
    int P = 2;
 
    int arr[] = { 3, 2, 0, 2, 7 };
 
    findArray(arr, N, P);
 
    return 0;
}

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Java

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// Java program to implement the
// above approach
import java.util.*;
 
class GFG{
 
// Function to generate and return the
// required initial arrangement
static void findArray(int []a, int n,
                               int P)
{
     
    // Store the minimum element
    // in the array
    int mi = Arrays.stream(a).min().getAsInt();
 
    // Store the number of increments
    int ctr = 0;
    mi = Math.max(0, mi - 1);
 
    // Subtract mi - 1 from every index
    for(int i = 0; i < n; i++)
    {
        a[i] -= mi;
        ctr += mi;
    }
 
    // Start from the last index which
    // had been incremented
    int i = P - 1;
 
    // Stores the index chosen to
    // distribute its element
    int start = -1;
 
    // Traverse the array cyclically and
    // find the index whose element was
    // distributed
    while (true)
    {
 
        // If any index has its
        // value reduced to 0
        if (a[i] == 0)
        {
 
            // Index whose element was
            // distributed
            start = i;
            break;
        }
        a[i] -= 1;
        ctr += 1;
        i = (i - 1 + n) % n;
    }
 
    // Store the number of increments
    // at the starting index
    a[start] = ctr;
 
    // Print the original array
    for(i = 0; i < n; i++)
    {
        System.out.print(a[i] + ", ");
    }
}
 
// Driver Code
public static void main(String[] args)
{
    int N = 5;
    int P = 2;
    int arr[] = { 3, 2, 0, 2, 7 };
 
    findArray(arr, N, P);
}
}
 
// This code is contributed by Rajput-Ji

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C#

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// C# program to implement the
// above approach
using System;
using System.Linq;
class GFG{
 
// Function to generate and return the
// required initial arrangement
static void findArray(int []a, int n,
                               int P)
{
     
    // Store the minimum element
    // in the array
    int mi = a.Min();
 
    // Store the number of increments
    int ctr = 0;
    mi = Math.Max(0, mi - 1);
 
    // Subtract mi - 1 from every index
    int i;
    for(i = 0; i < n; i++)
    {
        a[i] -= mi;
        ctr += mi;
    }
 
    // Start from the last index which
    // had been incremented
     i = P - 1;
 
    // Stores the index chosen to
    // distribute its element
    int start = -1;
 
    // Traverse the array cyclically and
    // find the index whose element was
    // distributed
    while (true)
    {
 
        // If any index has its
        // value reduced to 0
        if (a[i] == 0)
        {
 
            // Index whose element was
            // distributed
            start = i;
            break;
        }
        a[i] -= 1;
        ctr += 1;
        i = (i - 1 + n) % n;
    }
 
    // Store the number of increments
    // at the starting index
    a[start] = ctr;
 
    // Print the original array
    for(i = 0; i < n; i++)
    {
        Console.Write(a[i] + ", ");
    }
}
 
// Driver Code
public static void Main(String[] args)
{
    int N = 5;
    int P = 2;
    int []arr = { 3, 2, 0, 2, 7 };
 
    findArray(arr, N, P);
}
}
 
// This code is contributed by Rohit_ranjan

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Output: 

2, 1, 4, 1, 6,


 

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

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Improved By : Rajput-Ji, Rohit_ranjan