Minimize modulo operations to make given Array a permutation of [1, N]

Last Updated : 15 Feb, 2022

Given an array arr[] of size N, the task is to find the minimum number of operations required to make the array a permutation of numbers in range [1, N] where, in each operation, an element at any index i can be replaced by arr[i]%k (k = any value greater than 0). Return -1 if the array cannot be made a permutation of numbers in range [1, N].

Examples:

Input: arr [] = {1, 7}, N = 2
Output: 1
Explanation: The only possible sequence of operations which minimizes the no of operations is
Choose i = 1, k = 5. Perform arr[1] = arr[1] % 5 = 2.

Input: arr [] = {1,5,4}, N = 3
Output: -1
Explanation: It is impossible to obtain a permutation of integers from 1 to N.

Approach: The problem can be solved on the basis of the following observation.

x % y < (x/2) if x ≥ y, and
x % y = x if x < y.

As bigger is x, the longer the range of values that can be obtained after one mod operation.
So try to, assign smaller arr[i] to smaller numbers in the resulting permutation.

Although, if arr[i] satisfies 1 ≤ arr[i] ≤ N, just leave it there and use it in the resulting permutation.
if multiple arr[i] satisfy the same condition and have the same value, just choose one.
Let’s suppose in the optimal solution, l is changed to m and n to l for some n > l > m (l, m, n are values, not indices).
Then keeping l intact and changing n to m uses 1 less operation.
And, if it is possible to change n to l, then it must be possible to change n to m.

Follow the steps mentioned below to implement the approach:

Sort the array.
If 1 ≤ arr[i] ≤ N and it is the first occurrence of the element with value arr[i], leave it there.
Else, let the current least unassigned value in the resulting permutation be x:
- If x < arr[i]/2, assign the current element to value x and add the number of operations by 1.
- Else, output −1 directly.

Below is the implementation of the above approach.

C++

// C++ code to implement above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to find minimum operations
// used to make permutation of 1 to N
void minoperation(vector<int> arr, int N)
{
    set<int> st;
    vector<int> rem;
     
    // Storing one instance of every element 
    // from 1 to N
    for (int i = 1; i <= N; i++) {
        st.insert(i);
    }
 
    for (int i = 0; i < N; i++) {
        if (st.find(arr[i]) != st.end())
            st.erase(arr[i]);
        else
            rem.push_back(arr[i]);
    }
     
    // Sorting in descending order
    sort(rem.begin(), rem.end());
    reverse(rem.begin(), rem.end());
 
    int pt = 0;
    bool flag = false;
 
    for (auto& x : rem) {
        auto it = st.end();
        it--;
        int p = (*it);
        if (p > (x - 1) / 2) {
            flag = true;
            break;
        }
        st.erase(it);
    }
 
    if (flag) {
        // Not possible to make permutation.
        cout << "-1";
    }
    else {
        // Minimum number of operation required.
        cout << rem.size();
    }
}
 
// Driver code
int main()
{
    int N = 3;
    vector<int> arr = { 1, 5, 4 };
    minoperation(arr, N);
    return 0;
}

Java

// Java code to implement above approach
import java.util.*;
class GFG{
 
  // Function to find minimum operations
  // used to make permutation of 1 to N
  static void minoperation(int[] arr, int N)
  {
    HashSet<Integer> st = new HashSet<Integer>();
    Vector<Integer> rem = new Vector<Integer>();
 
    // Storing one instance of every element 
    // from 1 to N
    for (int i = 1; i <= N; i++) {
      st.add(i);
    }
 
    for (int i = 0; i < N; i++) {
      if (st.contains(arr[i]))
        st.remove(arr[i]);
      else
        rem.add(arr[i]);
    }
 
    // Sorting in descending order
    Collections.sort(rem,Collections.reverseOrder());
 
    boolean flag = false;
 
    for (int x : rem) {
      int it = st.size();
      it--;
      int p = new ArrayList<>(st).get(it);
      if (p > (x - 1) / 2) {
        flag = true;
        break;
      }
      st.remove(it);
    }
 
    if (flag) {
      // Not possible to make permutation.
      System.out.print("-1");
    }
    else {
      // Minimum number of operation required.
      System.out.print(rem.size());
    }
  }
 
  // Driver code
  public static void main(String[] args)
  {
    int N = 3;
    int[] arr = { 1, 5, 4 };
    minoperation(arr, N);
  }
}
 
// This code is contributed by 29AjayKumar

C#

// C# code to implement above approach
using System;
using System.Collections.Generic;
class GFG
{
 
  // Function to find minimum operations
  // used to make permutation of 1 to N
  static void minoperation(int[] arr, int N)
  {
    HashSet<int> st = new HashSet<int>();
    List<int> rem = new List<int>();
 
    // Storing one instance of every element 
    // from 1 to N
    for (int i = 1; i <= N; i++)
    {
      st.Add(i);
    }
 
    for (int i = 0; i < N; i++)
    {
      if (st.Contains(arr[i]))
        st.Remove(arr[i]);
      else
        rem.Add(arr[i]);
    }
 
    // Sorting in descending order
    rem.Sort();
    rem.Reverse();
 
    bool flag = false;
 
    foreach (int x in rem)
    {
      int it = st.Count;
      it--;
      int p = new List<int>(st)[it];
      if (p > (x - 1) / 2)
      {
        flag = true;
        break;
      }
      st.Remove(it);
    }
 
    if (flag)
    {
       
      // Not possible to make permutation.
      Console.Write("-1");
    }
    else
    {
       
      // Minimum number of operation required.
      Console.Write(rem.Count);
    }
  }
 
  // Driver code
  public static void Main()
  {
    int N = 3;
    int[] arr = { 1, 5, 4 };
    minoperation(arr, N);
  }
}
 
// This code is contributed by Saurabh jaiswal

Python3

# Python 3 code to implement above approach
 
# Function to find minimum operations
# used to make permutation of 1 to N
def minoperation(arr, N):
    st = set([])
    rem = []
 
    # Storing one instance of every element
    # from 1 to N
    for i in range(1, N + 1):
        st.add(i)
 
    for i in range(N):
        if (arr[i] in st):
            st.remove(arr[i])
 
        else:
            rem.append(arr[i])
 
    # Sorting in descending order
    rem.sort()
    rem.reverse()
 
    pt = 0
    flag = False
 
    for x in rem:
        it = len(st)
        it -= 1
        p = list(st)[it]
        if (p > (x - 1) / 2):
            flag = True
            break
 
        st.remove(it)
 
    if (flag):
        # Not possible to make permutation.
        print("-1")
 
    else:
        # Minimum number of operation required.
        print(len(rem))
 
# Driver code
if __name__ == "__main__":
 
    N = 3
    arr = [1, 5, 4]
    minoperation(arr, N)
 
    # This code is contributed by ukasp.

Javascript

<script>
        // JavaScript code for the above approach 
 
        // Function to find minimum operations
        // used to make permutation of 1 to N
        function minoperation(arr, N)
        {
            let st = new Set();
            let rem = [];
 
            // Storing one instance of every element 
            // from 1 to N
            for (let i = 1; i <= N; i++) {
                st.add(i);
            }
 
            for (let i = 0; i < N; i++) {
                if (st.has(arr[i]))
                    st.delete(arr[i]);
                else
                    rem.push(arr[i]);
            }
 
            // Sorting in descending order
            rem.sort(function (a, b) { return b - a })
            rem.reverse();
 
            let pt = 0;
            let flag = false;
 
            for (let x of rem) {
                let it = [...st].pop();
 
                let p = (it);
                if (p > Math.floor((x - 1) / 2)) {
                    flag = true;
                    break;
                }
                st.delete(it);
            }
 
            if (flag)
            {
             
                // Not possible to make permutation.
                document.write(-1)
            }
            else {
                // Minimum number of operation required.
                document.write(rem.size)
            }
        }
 
        // Driver code
        let N = 3;
        let arr = [1, 5, 4];
        minoperation(arr, N);
 
  // This code is contributed by Potta Lokesh
    </script>