[leetcode-54-Spiral Matrix]

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

思路：

上下左右4个变量分别表示边界，如果越界，结束循环。

vector<int> spiralOrder(vector<vector<int>>& matrix)
     {
         if (matrix.empty())return{};
         int m = matrix.size(), n = matrix[0].size();
         vector<int> spiral(m*n);
         int up = 0, down = m - 1, left = 0, right = n - 1, k = 0;
         while (1)
         {
             for (int col = left; col <= right; col++)spiral[k++] = matrix[up][col];
             if (++up > down)break;
             for (int row = up; row <= down; row++)spiral[k++] = matrix[row][right];
             if (--right < left)break;
             for (int col = right; col >= left; col--)spiral[k++] = matrix[down][col];
             if (--down < up)break;
             for (int row = down; row >= up; row--)spiral[k++] = matrix[row][left];
             if (++left > right)break;              
         }
         return spiral;
    }

参考：

https://discuss.leetcode.com/topic/21090/0ms-clear-c-solution