Python 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. 

Python 3

# Python 3 program to print
# matrix in anti-spiral form
R = 4
C = 5

def antiSpiralTraversal(m, n, a):
    k = 0
    l = 0

    # k - starting row index
    # m - ending row index
    # l - starting column index
    # n - ending column index
    # i - iterator
    stk = []

    while (k <= m and l <= n):

        # Print the first row
        # from the remaining rows
        for i in range(l, n + 1):
            stk.append(a[k][i])
        k += 1

        # Print the last column
        # from the remaining columns
        for i in range(k, m + 1):
            stk.append(a[i][n])
        n -= 1

        # Print the last row
        # from the remaining rows
        if ( k <= m):
            for i in range(n, l - 1, -1):
                stk.append(a[m][i])
            m -= 1

        # Print the first column
        # from the remaining columns
        if (l <= n):
            for i in range(m, k - 1, -1):
                stk.append(a[i][l])
            l += 1

    while len(stk) != 0:
        print(str(stk[-1]), end = " ")
        stk.pop()

# Driver Code
mat = [[1, 2, 3, 4, 5],
       [6, 7, 8, 9, 10],
       [11, 12, 13, 14, 15],
       [16, 17, 18, 19, 20]];

antiSpiralTraversal(R - 1, C - 1, mat)

# This code is contributed
# by ChitraNayal

Output:

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

Time complexity: O(R + C).
Auxiliary Space: O(R + C).

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