59. Spiral Matrix II

Simulation

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
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

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
class Solution {
    public int[][] generateMatrix(int n) {
        int[][] matrix = new int[n][n];
        int value = 1;

        int rowStartIndex = 0, rowEndIndex = n - 1, colStartIndex = 0, colEndIndex = n - 1;
        while (true) {
            // left -> right
            for (int colIndex = colStartIndex; colIndex <= colEndIndex; colIndex++) {
                matrix[rowStartIndex][colIndex] = value++;
            }
            if (++rowStartIndex > rowEndIndex) {
                break;
            }

            // top -> bottom
            for (int rowIndex = rowStartIndex; rowIndex <= rowEndIndex; rowIndex++) {
                matrix[rowIndex][colEndIndex] = value++;
            }
            if (--colEndIndex < colStartIndex) {
                break;
            }

            // right -> left
            for (int colIndex = colEndIndex; colIndex >= colStartIndex; colIndex--) {
                matrix[rowEndIndex][colIndex] = value++;
            }
            if (--rowEndIndex < rowStartIndex) {
                break;
            }

            // bottom -> top
            for (int rowIndex = rowEndIndex; rowIndex >= rowStartIndex; rowIndex--) {
                matrix[rowIndex][colStartIndex] = value++;
            }
            if (++colStartIndex > colEndIndex) {
                break;
            }
        }

        return matrix;
    }
}

References

59. Spiral Matrix II