顺时针打印二维矩阵

public ArrayList printMatrix(int[][] matrix) {
    ArrayList result = new ArrayList();
    if (matrix == null || matrix.length == 0 || matrix[0].length == 0) {
        return result;
    }
    int left = 0;
    int right = matrix[0].length - 1;
    int top = 0;
    int bottom = matrix.length - 1;
    while (true) {
        //上边界
        for (int col = left; col <= right; col++) {
            result.add(matrix[top][col]);
        }
        top++;
        if (top > bottom) break;

        //右边界
        for (int row = top; row <= bottom; row++) {
            result.add(matrix[row][right]);
        }
        right--;
        if (left > right) break;

        //下边界
        for (int col = right; col >= left; col--) {
            result.add(matrix[bottom][col]);
        }
        bottom--;
        if (top > bottom) break;

        //左边界
        for (int row = bottom; row >= top; row--) {
            result.add(matrix[row][left]);
        }
        left++;
        if (left > right) break;
    }
    return result;
}