Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

For example,
Given the following matrix:

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[
 [ 1, 2, 3 ],
 [ 4, 5, 6 ],
 [ 7, 8, 9 ]
]

You should return [1,2,3,6,9,8,7,4,5].

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public class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
        List<Integer> res = new LinkedList<>();
        if (matrix == null || matrix.length == 0) return res;
        int rowEnd = matrix.length - 1, columnEnd = matrix[0].length - 1, rowBegin = 0, columnBegin = 0;
        
        while (rowBegin <= rowEnd && columnBegin <= columnEnd) {
        	// top, left to right
            for (int i = columnBegin; i <= columnEnd; i++) {
                res.add(matrix[rowBegin][i]);
            }
            rowBegin++;
            // right, top to bottom
            for (int j = rowBegin; j <= rowEnd; j++) {
                res.add(matrix[j][columnEnd]);
            }
            columnEnd--;
            // bottom, right to left
            if (rowBegin <= rowEnd) {
                for (int k = columnEnd; k >= columnBegin; k--) {
                    res.add(matrix[rowEnd][k]);
                }
            }
            
            // left, bottom to top
            rowEnd--;
            if (columnBegin <= columnEnd) {
                for (int l = rowEnd; l >= rowBegin; l--) {
                    res.add(matrix[l][columnBegin]);
                }
            }
            columnBegin++;
        }
        return res;
    }
}