54. 螺旋矩阵

给你一个 m 行 n 列的矩阵 matrix ，请按照 顺时针螺旋顺序 ，返回矩阵中的所有元素。

示例 1：

输入：matrix = [[1,2,3],[4,5,6],[7,8,9]]
输出：[1,2,3,6,9,8,7,4,5]
示例 2：

输入：matrix = [[1,2,3,4],[5,6,7,8],[9,10,11,12]]
输出：[1,2,3,4,8,12,11,10,9,5,6,7]

class Solution {
public:
    vector<int> spiralOrder(vector<vector<int>>& matrix) {
        vector<int> res;
        int left = 0,up = 0,right = matrix[0].size()-1,down = matrix.size()-1;
        while(left<=right&&up<=down){
             for(int j = left;j<=right&&up<=down;j++){
              res.push_back(matrix[up][j]);
            }
            up++;
            for(int i = up;i<=down&&left<=right;i++){
                res.push_back(matrix[i][right]);
            }
            right--;
            for(int j = right;j>=left&&up<=down;j--){
                res.push_back(matrix[down][j]);
            }
            down--;
            for(int i = down;i>=up&&left<=right;i--){
                res.push_back(matrix[i][left]);
            }
            left++;
        }
        return res;
    }
};