Matrix in Reverse Spiral Form

Program Code in C++

#include <iostream>
using namespace std;

void ReverseSpiral(int m, int n, int a[r][c])
{
long int input[100];
int i, x = 0, y= 0; // x - starting row index and y- starting column index
int z = 0; // Counter for the 1D array that will store the spiral form 
int size = m*n; //Total number of elements
while (x < m && y < n)
{
	int val;
	for (i = y; i < n; ++i)
	{
	// printf("%d ", a[x][i]);
		val = a[x][i];
		input[z] = val;
		++z;
	}
x++;


/* Print the last column from the remaining columns */
for (i = x; i < m; ++i)
{
	// printf("%d ", a[i][n-1]);
	val = a[i][n-1];
	input[z] = val;
	++z;
}
n--;


if ( x < m)
{
	for (i = n-1; i >= y; --i)
	{
		printf("%d ", a[m-1][i]);
		val = a[m-1][i];
		input[z] = val;
		++z;
	}
m--;
}


if (y < n)
{
	for (i = m-1; i >= x; --i)
	{
		// printf("%d ", a[i][x]);
		val = a[i][l];
		b[z] = val;
		++z;
	}
y++;
}
}
for (int i=size-1 ; i>=0 ; --i)
{
	cout<<input[i]<<" ";
}
}
int main()
{
int a[r][c] = { {1, 2, 3, 4},{12,13,14,5},{11,16,15,6},{10,9,8,7}};
ReverseSpiral(r, c, a);
return 0;
}

You can also try this code with Online C++ Compiler

Run Code

Output 

16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

Complexity Analysis

Time Complexity: O(n) as the for loop runs for n number of times.

Space Complexity: O(n) as the array int a is being taken which occupies space in the memory.

Check out this problem - Reverse Nodes In K Group
Must Read  C Program to Reverse a Number

Frequently Asked Questions

Is it necessary that the input matrix should be a two-dimensional one?

Yes, it is necessary that the input matrix to find out the reverse spiral matrix should be a 2D matrix only because otherwise, we won’t be able to form a spiral in the case of a 1D matrix.

Where will the spiral form start from?

For finding the spiral form of the input matrix, we will always be starting with the topmost left corner element, and then we move clockwise.

Conclusion

This article extensively discussed the problem of printing the reverse spiral form of a matrix. Spiral form, as the name suggests, is formed by forming a spiral around a matrix starting from the top leftmost element and moving to the center of the matrix. 

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