BM98 螺旋矩阵 C++ 详细注释

https://www.nowcoder.com/practice/7edf70f2d29c4b599693dc3aaeea1d31
思路：按右、下、左、上的方向遍历矩阵，注意边界判断

class Solution {
public:
    vector spiralOrder(vector > &matrix) {
        vector ans;
        if (!matrix.empty()) {
            // m×n矩阵
            int m = matrix.size(), n = matrix[0].size();
            // 上、右、下、左 边界
            int top = -1, right = n, down = m, left = -1;
            // 遍历下标
            int i = 0, j = 0;

            while (true) {
                // 往右走
                while (j < right) {
                    ans.push_back(matrix[i][j++]);
                }
                // 更新边界
                i++;j--;top++;
                // 边界判断
                if (top >= down-1) break;
                // 往下走
                while (i < down) {
                    ans.push_back(matrix[i++][j]);
                }
                i--;j--;right--;
                if (left >= right-1) break;
                // 往左走
                while (j > left) {
                    ans.push_back(matrix[i][j--]);
                }
                i--;j++;down--;
                if (top >= down-1) break;
                // 往上走
                while (i > top) {
                    ans.push_back(matrix[i--][j]);
                }
                i++;j++;left++;
                if (left >= right-1) break;
            }
        }
        return ans;
    }
};