54 Spiral Matrix

Given a matrix of_m_x_n_elements (_m_rows,_n_columns), return all elements of the matrix in spiral order.

Example 1:

Input:

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

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

Example 2:

Input:

[
  [1, 2, 3, 4],
  [5, 6, 7, 8],
  [9,10,11,12]
]

Output:
 [1,2,3,4,8,12,11,10,9,5,6,7]

Solution)

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