769. 螺旋矩阵

版权声明：世界是开源的 https://blog.csdn.net/qq_36619282/article/details/81983138

描述：给出整数 n, 返回一个大小为 n * n 的螺旋矩阵

给出 n = 3

则螺旋矩阵为:

[ [1,2,3]，[8,9,4]，[7,6,5] ]

public class Solution {
    /**
     * @param n: a Integer
     * @return: a spiral array
     */
    public int[][] spiralArray(int n) {
        int[][] res = new int[n][n];
                      // right, down, left, up
        int[] dx = new int[]{0, 1, 0, -1}; // row index's change in 4 directions
        int[] dy = new int[]{1, 0, -1, 0}; // column index's change in 4 directions

        // currently: x, y
        // direction d
        // next grid: (x + dx[d], y + dy[d])

        int x, y, d;
        int i, j, nx, ny;
        // initialization
        for (i = 0; i < n; ++i) {
            for (j = 0; j < n; ++j) {
                res[i][j] = -1; // (i,j) has not been filled yet
            }
        }

        x = 0;
        y = 0;
        d = 0;
        for (i = 1; i <= n * n; ++i) {
            res[x][y] = i; // fill in i at (x, y)
            // explore
            nx = x + dx[d];
            ny = y + dy[d];
            // next grid along direction d is (nx, ny)
            // rorate if it's outside of the matrix or the grid has been filled
            if (nx < 0 || nx >= n || ny < 0 || ny >= n || res[nx][ny] != -1) {
                // rotate
                // direction d: 0-->1, 1-->2, 2-->3, 3-->0
                d = (d + 1) % 4;
                nx = x + dx[d];
                ny = y + dy[d];
            }

            x = nx;
            y = ny;
        }

        return res;
    }
}