59. Spiral Matrix II

Simulation

class Solution {
    public int[][] generateMatrix(int n) {
        int[][] matrix = new int[n][n];

        int[][] directions = new int[][]{{0, 1}, {1, 0}, {0, -1}, {-1, 0}};
        int directionIndex = 0;

        int topIndex = 0, downIndex = n - 1, leftIndex = 0, rightIndex = n - 1;

        int i = 0, j = 0;
        for (int k = 1; k <= n * n; k++) {
            matrix[i][j] = k;

            int nextI = i + directions[directionIndex][0];
            int nextJ = j + directions[directionIndex][1];

            // 注意此处巧妙利用了下一个格子是否为 0 来判断是否到达了已经填充过的边界
            if (nextJ > rightIndex || nextI > downIndex || nextJ < leftIndex || nextI < topIndex || matrix[nextI][nextJ] != 0) {
                directionIndex = (directionIndex + 1) % 4;
            }

            i += directions[directionIndex][0];
            j += directions[directionIndex][1];
        }

        return matrix;
    }
}

Simulation

class Solution {
    public int[][] generateMatrix(int n) {
        int[][] matrix = new int[n][n];

        int topIndex = 0, downIndex = n - 1, leftIndex = 0, rightIndex = n - 1;
        int num = 1, maxNum = n * n;
        while (num <= maxNum) {
            for (int j = leftIndex; j <= rightIndex; j++) {
                matrix[topIndex][j] = num++;
            }
            topIndex++;
            for (int i = topIndex; i <= downIndex; i++) {
                matrix[i][rightIndex] = num++;
            }
            rightIndex--;
            for (int j = rightIndex; j >= leftIndex; j--) {
                matrix[downIndex][j] = num++;
            }
            downIndex--;
            for (int i = downIndex; i >= topIndex; i--) {
                matrix[i][leftIndex] = num++;
            }
            leftIndex++;
        }

        return matrix;
    }
}

Reference

59. Spiral Matrix II