LeetCode - Spiral Matrix II | Darren's Blog

Given an integer n , generate a square matrix filled with elements from 1 to n2  in spiral order

For example,
Given n = 3,

You should return the following matrix:

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

public int[][] generateMatrix(int n) {

        if (n < 0)

            return null;

        int[][] matrix = new int[n][n];

        int startRow = 0, startColumn = 0;

        int endRow = n-1, endColumn = n-1;

        int num = 1;

        // Work on 2-D submatrix ranging from Row startRow to endRow,

        // Column startColumn to endColumn

        while (startRow < endRow && startColumn < endColumn) {

            // Top edge, left-to-right

            for (int k = startColumn; k < endColumn; k++)

                matrix[startRow][k] = num++;

            // Right edge, top-to-bottom

            for (int k = startRow; k < endRow; k++)

                matrix[k][endColumn] = num++;

            // Bottom edge, right-to-left

            for (int k = endColumn; k > startColumn; k--)

                matrix[endRow][k] = num++;

            // Left edge, bottom-to-top

            for (int k = endRow; k > startRow; k--)

                matrix[k][startColumn] = num++;

            // Prepare to work on its nested submatrix

            startRow++;

            startColumn++;

            endRow--;

            endColumn--;

        }

        // When n is odd, there is still one cell at the center of the matrix

        // that has not been assigned a value

        if ((n & 1) == 1)

            matrix[startRow][startColumn] = num;

 

        return matrix;

    }

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