class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
        List<Integer> result = new ArrayList<>();
        if(matrix == null || matrix.length == 0) return result;
        int top = 0, bottom = matrix.length - 1;
        int left = 0, right = matrix[0].length - 1;
        while(top <= bottom && left <= right) {
            for(int i = left; i <= right; i++) result.add(matrix[top][i]);
            top++;
            for(int i = top; i <= bottom; i++) result.add(matrix[i][right]);
            right--;
            if(top <= bottom) {
                for(int i = right; i >= left; i--) result.add(matrix[bottom][i]);
                bottom--;
            }
            if(left <= right) {
                for(int i = bottom; i >= top; i--) result.add(matrix[i][left]);
                left++;
            }
        }
        return result;
    }
}