Solution.java

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