54. Spiral Matrix

class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
        List<Integer> result = new ArrayList<>();
        if (matrix == null || matrix.length == 0){
            return result;
        }
        int m = matrix.length;
        int n = matrix[0].length;
        int top = 0;
        int bottom = m-1;
        int left = 0;
        int right = n-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;
    }
}