leetcode_gym/_problems/spiralMatrix.md

layout, title, last_solved, category, leetcode_url, status

layout	title	last_solved	category	leetcode_url	status
page	Spiral Matrix	2020-07-16	matrix	https://leetcode.com/problems/spiral-matrix/	Attempted

Problem

Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

Example 1:

Input:
[
 [ 1, 2, 3 ],
 [ 4, 5, 6 ],
 [ 7, 8, 9 ]
]
Output: [1,2,3,6,9,8,7,4,5]
Example 2:

Input:
[
  [1, 2, 3, 4],
  [5, 6, 7, 8],
  [9,10,11,12]
]
Output: [1,2,3,4,8,12,11,10,9,5,6,7]

Solution

This is a really good problem to practice reverse for loops and matrix traversal

{% highlight python %}

matrix = [ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ] ]

def spiralMatrix(matrix):

start_row = 0
end_row = len(matrix)
start_col = 0
end_col = len(matrix)

output = []
while start_row<end_row or start_col<end_col:
    # right
    for i in range(start_col, end_col):
        output.append( matrix[start_row][i] )

    start_row+=1

    # down
    for i in range(start_row, end_row):
        output.append( matrix[i][end_col-1] )

    end_col-=1

    # left
    for i in range(end_col-1, start_col-1, -1):
        output.append( matrix[end_row-1][i] )

    end_row-=1

    # up
    for i in range(end_row-1, start_row-1, -1):
        output.append( matrix[i][start_col] )

    start_col+=1

return output

print( spiralMatrix(matrix) )

{% endhighlight %}

---

Dealing with the "edge cases" proved a bit difficult. Would need to analyze this a bit further:

{% highlight python %}

matrix1 = [ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ] ]

matrix2 = [ [1, 2, 3, 4], [4,5,6,7], [7,8,9,10] ]

matrix3 = [ [2,5,8],[4,0,-1] ]

def spiralMatrix1(matrix):

if not matrix: return []

start_row = 0
end_row = len(matrix)
start_col = 0
end_col = len(matrix[0])

output = []
while start_row<end_row and start_col<end_col:      # also note the 'or' changed to an 'and'
    # right
    for i in range(start_col, end_col):
        output.append( matrix[start_row][i] )

    start_row+=1

    # down
    for i in range(start_row, end_row):
        output.append( matrix[i][end_col-1] )


    end_col-=1

    if end_row <= start_row: break  # handles edge case with matrix2

    # left
    for i in range(end_col-1, start_col-1, -1):
        output.append( matrix[end_row-1][i] )

    end_row-=1

    if end_col <= start_col: break  # handles edge case with matrix2

    # up
    for i in range(end_row-1, start_row-1, -1):
        output.append( matrix[i][start_col] )

    start_col+=1

return output

print( spiralMatrix1(matrix3) )

{% endhighlight %}