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Construct a matrix such that union of ith row and ith column contains every element from 1 to 2N-1
  • Difficulty Level : Hard
  • Last Updated : 28 May, 2021

Given a number N, the task is to construct a square matrix of N * N where union of elements in some ith row with the ith column contains every element in the range [1, 2*N-1]. If no such matrix exists, print -1.
Note: There can be multiple possible solutions for a particular N
Examples: 
 

Input: N = 6 
Output: 
6 4 2 5 3 1 
10 6 5 3 1 2 
8 11 6 1 4 3 
11 9 7 6 2 4 
9 7 10 8 6 5 
7 8 9 10 11 6 
Explanation: 
The above matrix is of 6 * 6 in which every ith row and column contains elements from 1 to 11, like: 
1st row and 1st column = {6, 4, 2, 5, 3, 1} & {6, 10, 8, 11, 9, 7} = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11} 
2nd row and 2nd column = {10, 6, 5, 3, 1, 2} & {4, 6, 11, 9, 7, 8} = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11} 
3rd row and 3rd column = {8, 11, 6, 1, 4, 3} & {2, 5, 6, 7, 10, 9} = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11} 
4th row and 4th column = {11, 9, 7, 6, 2, 4} & {5, 3, 1, 6, 8, 10} = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11} 
5th row and 5th column = {9, 7, 10, 8, 6, 5} & {3, 1, 4, 2, 6, 11} = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11} 
6th row and 6th column = {7, 8, 9, 10, 11, 6} & {1, 2, 3, 4, 5, 6} = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}
Input: N = 6 
Output: -1 
Explanation: 
There is no such matrix possible in which every i’th row and i’th column contains every elements from 1 to 9. Hence, the answer is -1. 
 

 

Approach: 
If we observe carefully, we can see that: 
 

  • For any odd number except 1, the square matrix is not possible to generate
  • To generate the square matrix for even order, the idea is to fill up the upper half of the diagonal elements of the matrix in the range of 1 to N-1 and fill all the diagonal elements with the N and the lower half of the diagonal elements can be filled in number from range N + 1 to 2N – 1.

Below is the algorithm fr this approach: 
 



  1. Matrix of odd order can’t be filled as observed except for N = 1
  2. For Matrix of even order, 
    • Firstly, fill all diagonal elements equal to N.
    • Consider the two halves of the matrix diagonally bisected, each half can be filled with N-1 elements.
    • Fill upper half with elements from [1, N-1] and the lower half with elements from [N+1, 2N-1].
    • As it can be easily observed that there is a pattern that second row’s last element can be always 2.
    • Now, consecutive elements in the last column are at a difference of 2. Hence generalised form can be given as A[i]=[(N-2)+2i]%(N-1)+1, for all i from 1 to N-1
    • Simply add N to all the elements of the lower half.

Below is the implementation of the above approach:
 

C++




// C++ implementation of the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
int matrix[100][100];
 
// Function to find the square matrix
void printRequiredMatrix(int n)
{
    // For Matrix of order 1,
    // it will contain only 1
    if (n == 1) {
        cout << "1"
             << "\n";
    }
 
    // For Matrix of odd order,
    // it is not possible
    else if (n % 2 != 0) {
        cout << "-1"
             << "\n";
    }
 
    // For Matrix of even order
    else {
        // All diagonal elements of the
        // matrix can be N itself.
        for (int i = 0; i < n; i++) {
            matrix[i][i] = n;
        }
        int u = n - 1;
 
        // Assign values at desired
        // place in the matrix
        for (int i = 0; i < n - 1; i++) {
 
            matrix[i][u] = i + 1;
 
            for (int j = 1; j < n / 2; j++) {
 
                int a = (i + j) % (n - 1);
                int b = (i - j + n - 1) % (n - 1);
                if (a < b)
                    swap(a, b);
                matrix[b][a] = i + 1;
            }
        }
 
        // Loop to add N in the lower half
        // of the matrix such that it contains
        // elements from 1 to 2*N - 1
        for (int i = 0; i < n; i++)
            for (int j = 0; j < i; j++)
                matrix[i][j] = matrix[j][i] + n;
 
        // Loop to print the matrix
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++)
                cout << matrix[i][j] << " ";
            cout << "\n";
        }
    }
    cout << "\n";
}
 
// Driver Code
int main()
{
    int n = 1;
    printRequiredMatrix(n);
 
    n = 3;
    printRequiredMatrix(n);
 
    n = 6;
    printRequiredMatrix(n);
 
    return 0;
}

Java




// Java implementation of the above approach
class GFG {
     
    static int matrix[][] = new int[100][100];
     
    // Function to find the square matrix
    static void printRequiredMatrix(int n)
    {
        // For Matrix of order 1,
        // it will contain only 1
        if (n == 1) {
            System.out.println("1");
        }
     
        // For Matrix of odd order,
        // it is not possible
        else if (n % 2 != 0) {
            System.out.println("-1");
        }
     
        // For Matrix of even order
        else {
            // All diagonal elements of the
            // matrix can be N itself.
            for (int i = 0; i < n; i++) {
                matrix[i][i] = n;
            }
            int u = n - 1;
     
            // Assign values at desired
            // place in the matrix
            for (int i = 0; i < n - 1; i++) {
     
                matrix[i][u] = i + 1;
     
                for (int j = 1; j < n / 2; j++) {
     
                    int a = (i + j) % (n - 1);
                    int b = (i - j + n - 1) % (n - 1);
                    if (a < b) {
                        int temp = a;
                        a = b;
                        b = temp;
                    }
                    matrix[b][a] = i + 1;
                }
            }
     
            // Loop to add N in the lower half
            // of the matrix such that it contains
            // elements from 1 to 2*N - 1
            for (int i = 0; i < n; i++)
                for (int j = 0; j < i; j++)
                    matrix[i][j] = matrix[j][i] + n;
     
            // Loop to print the matrix
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < n; j++)
                    System.out.print(matrix[i][j] + " ");
                System.out.println() ;
            }
        }
    System.out.println();
    }
     
    // Driver Code
    public static void main (String[] args)
    {
        int n = 1;
        printRequiredMatrix(n);
     
        n = 3;
        printRequiredMatrix(n);
     
        n = 6;
        printRequiredMatrix(n);
     
    }
}
 
// This code is contributed by AnkitRai01

Python3




# Python3 implementation of the above approach
import numpy as np;
 
matrix = np.zeros((100,100));
 
# Function to find the square matrix
def printRequiredMatrix(n) :
 
    # For Matrix of order 1,
    # it will contain only 1
    if (n == 1) :
        print("1");
 
    # For Matrix of odd order,
    # it is not possible
    elif (n % 2 != 0) :
        print("-1");
 
    # For Matrix of even order
    else :
        # All diagonal elements of the
        # matrix can be N itself.
        for i in range(n) :
            matrix[i][i] = n;
     
        u = n - 1;
 
        # Assign values at desired
        # place in the matrix
        for i in range(n - 1) :
 
            matrix[i][u] = i + 1;
 
            for j in range(1, n//2) :
 
                a = (i + j) % (n - 1);
                b = (i - j + n - 1) % (n - 1);
                if (a < b) :
                    a,b = b,a
                     
                matrix[b][a] = i + 1;
 
        # Loop to add N in the lower half
        # of the matrix such that it contains
        # elements from 1 to 2*N - 1
        for i in range(n) :
            for j in range(i) :
                matrix[i][j] = matrix[j][i] + n;
 
        # Loop to print the matrix
        for i in range(n) :
            for j in range(n) :
                print(matrix[i][j] ,end=" ");
            print();
 
    print()
 
# Driver Code
if __name__ == "__main__" :
 
    n = 1;
    printRequiredMatrix(n);
 
    n = 3;
    printRequiredMatrix(n);
 
    n = 6;
    printRequiredMatrix(n);
 
    # This code is contributed by AnkitRai01

C#




// C# implementation of the above approach
using System;
 
class GFG {
     
    static int [,]matrix = new int[100, 100];
     
    // Function to find the square matrix
    static void printRequiredMatrix(int n)
    {
        // For Matrix of order 1,
        // it will contain only 1
        if (n == 1) {
            Console.WriteLine("1");
        }
     
        // For Matrix of odd order,
        // it is not possible
        else if (n % 2 != 0) {
            Console.WriteLine("-1");
        }
     
        // For Matrix of even order
        else
        {
            // All diagonal elements of the
            // matrix can be N itself.
            for (int i = 0; i < n; i++) {
                matrix[i, i] = n;
            }
            int u = n - 1;
     
            // Assign values at desired
            // place in the matrix
            for (int i = 0; i < n - 1; i++) {
     
                matrix[i, u] = i + 1;
     
                for (int j = 1; j < n / 2; j++) {
     
                    int a = (i + j) % (n - 1);
                    int b = (i - j + n - 1) % (n - 1);
                    if (a < b) {
                        int temp = a;
                        a = b;
                        b = temp;
                    }
                    matrix[b, a] = i + 1;
                }
            }
     
            // Loop to add N in the lower half
            // of the matrix such that it contains
            // elements from 1 to 2*N - 1
            for (int i = 0; i < n; i++)
                for (int j = 0; j < i; j++)
                    matrix[i, j] = matrix[j, i] + n;
     
            // Loop to print the matrix
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < n; j++)
                    Console.Write(matrix[i, j] + " ");
                Console.WriteLine() ;
            }
        }
    Console.WriteLine();
    }
     
    // Driver Code
    public static void Main (String[] args)
    {
        int n = 1;
        printRequiredMatrix(n);
     
        n = 3;
        printRequiredMatrix(n);
     
        n = 6;
        printRequiredMatrix(n);
    }
}
 
// This code is contributed by Yash_R

Javascript




<script>
// Javascript implementation of the above approach
 
let matrix = new Array();
 
for(let i = 0;  i < 100; i++){
    let temp = new Array();
    for(let j = 0; j < 100; j++){
        temp.push([])
    }
    matrix.push(temp)
}
 
// Function to find the square matrix
function printRequiredMatrix(n)
{
    // For Matrix of order 1,
    // it will contain only 1
    if (n == 1) {
        document.write("1" + "<br>");
    }
 
    // For Matrix of odd order,
    // it is not possible
    else if (n % 2 != 0) {
        document.write("-1" + "<br>");
    }
 
    // For Matrix of even order
    else {
        // All diagonal elements of the
        // matrix can be N itself.
        for (let i = 0; i < n; i++) {
            matrix[i][i] = n;
        }
        let u = n - 1;
 
        // Assign values at desired
        // place in the matrix
        for (let i = 0; i < n - 1; i++) {
 
            matrix[i][u] = i + 1;
 
            for (let j = 1; j < n / 2; j++) {
 
                let a = (i + j) % (n - 1);
                let b = (i - j + n - 1) % (n - 1);
                if (a < b){
                    let temp = a;
                    a = b;
                    b = temp
                }
                matrix[b][a] = i + 1;
            }
        }
 
        // Loop to add N in the lower half
        // of the matrix such that it contains
        // elements from 1 to 2*N - 1
        for (let i = 0; i < n; i++)
            for (let j = 0; j < i; j++)
                matrix[i][j] = matrix[j][i] + n;
 
        // Loop to print the matrix
        for (let i = 0; i < n; i++) {
            for (let j = 0; j < n; j++)
                document.write(matrix[i][j] + " ");
            document.write("<br>");
        }
    }
    document.write("<br>");
}
 
// Driver Code
 
let n = 1;
printRequiredMatrix(n);
 
n = 3;
printRequiredMatrix(n);
 
n = 6;
printRequiredMatrix(n);
 
// This code is contributed by gfgking
</script>
Output: 
1

-1

6 4 2 5 3 1 
10 6 5 3 1 2 
8 11 6 1 4 3 
11 9 7 6 2 4 
9 7 10 8 6 5 
7 8 9 10 11 6

 

Performance Analysis: 
 

  • Time Complexity: As in the above approach, there is two loops to iterate over the whole N*N matrix which takes O(N2) time in worst case, Hence the Time Complexity will be O(N2).
  • Auxiliary Space Complexity: As in the above approach, there is no extra space used, Hence the auxiliary space complexity will be O(1)

 

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