Java Program to Print matrix in antispiral form

Given a 2D array, the task is to print matrix in anti spiral form:
Examples:

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

Input : arr[][4] = {1, 2, 3, 4
                    5, 6, 7, 8
                    9, 10, 11, 12
                    13, 14, 15, 16};
Output : 10 11 7 6 5 9 13 14 15 16 12 8 4 3 2 1

Input :arr[][6] = {1, 2, 3, 4, 5, 6
                  7, 8, 9, 10, 11, 12
                  13, 14, 15, 16, 17, 18};
Output : 11 10 9 8 7 13 14 15 16 17 18 12 6 5 4 3 2 1

The idea is simple, we traverse matrix in spiral form and put all traversed elements in a stack. Finally one by one elements from stack and print them. 

Java

// Java Code for Print matrix in antispiral form
import java.util.*;

class GFG {

    public static void antiSpiralTraversal(int m, int n,
                                             int a[][])
    {
        int i, k = 0, l = 0;

        /*  k - starting row index
            m - ending row index
            l - starting column index
            n - ending column index
            i - iterator  */
        Stack<Integer> stk=new Stack<Integer>();

        while (k <= m && l <= n)
        {
            /* Print the first row from the remaining
             rows */
            for (i = l; i <= n; ++i)
                stk.push(a[k][i]);
            k++;

            /* Print the last column from the remaining
            columns */
            for (i = k; i <= m; ++i)
                stk.push(a[i][n]);
            n--;

            /* Print the last row from the remaining
            rows */
            if ( k <= m)
            {
                for (i = n; i >= l; --i)
                    stk.push(a[m][i]);
                m--;
            }

            /* Print the first column from the remaining
            columns */
            if (l <= n)
            {
                for (i = m; i >= k; --i)
                    stk.push(a[i][l]);
                l++;
            }
        }

        while (!stk.empty())
        {
            System.out.print(stk.peek() + " ");
            stk.pop();
        }
    }

    /* Driver program to test above function */
    public static void main(String[] args)
    {
         int mat[][] =
                {
                    {1,  2,  3,  4,  5},
                    {6,  7,  8,  9,  10},
                    {11, 12, 13, 14, 15},
                    {16, 17, 18, 19, 20}
                };

        antiSpiralTraversal(mat.length - 1, mat[0].length - 1,
                                                       mat);
    }
  }
// This code is contributed by Arnav Kr. Mandal.

Output:

12 13 14 9 8 7 6 11 16 17 18 19 20 15 10 5 4 3 2 1 

Time Complexity: O(m x n) where m is the number of rows and n is the number of columns in the matrix.

Space Complexity: O(m x n) as we are using a stack to store the elements.

Please refer complete article on Print matrix in antispiral form for more details!