Spiral Matrix III — LeetCode 885 Python Solution

The problem

You start at the cell (rStart, cStart) of an rows x cols grid facing east. The northwest corner is at the first row and column in the grid, and the southeast corner is at the last row and column.

Example

Input

rows = 1, cols = 4, rStart = 0, cStart = 0

Output

[[0,0],[0,1],[0,2],[0,3]]

Python solution

class Solution:
    def spiralMatrixIII(
        self, rows: int, cols: int, rStart: int, cStart: int
    ) -> List[List[int]]:
        ans = [[rStart, cStart]]
        if rows * cols == 1:
            return ans
        k = 1
        while True:
            for dr, dc, dk in [[0, 1, k], [1, 0, k], [0, -1, k + 1], [-1, 0, k + 1]]:
                for _ in range(dk):
                    rStart += dr
                    cStart += dc
                    if 0 <= rStart < rows and 0 <= cStart < cols:
                        ans.append([rStart, cStart])
                        if len(ans) == rows * cols:
                            return ans
            k += 2

Complexity

Pattern: Matrix and Grid

Treat a 2-D grid as a graph whose neighbours are the four adjacent cells. LeetCode 885. Spiral Matrix III is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Matrix.

The matrix and grid guide has the Python template for the pattern and the 216 LeetCode problems that use it.

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