19. 顺时针打印矩阵

输入一个矩阵，按照从外向里以顺时针的顺序依次打印出每一个数字，例如，如果输入如下4 X 4矩阵： 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 则依次打印出数字1,2,3,4,8,12,16,15,14,13,9,5,6,7,11,10.

思路：只需要记录四个边界，然后按照逆时针顺序打印结果就可。注意条件是，左边界 <= 右边界 && 上边界 <= 下边界

class Solution {
public:
    vector<int> printMatrix(vector<vector<int> > matrix) {
        vector<int> res;
        if (matrix.empty()) return res;
        int rows = matrix.size(), cols = matrix[0].size();
        int up = 0, down = rows - 1, left = 0, right = cols - 1;
        while(up <= down && left <= right) {
            // top line
            for (int i = left; i <= right; ++i)
                res.push_back(matrix[up][i]);
            ++up;
            // right line
            if (up <= down && left <= right) {
                for (int i = up; i <= down; ++i)
                    res.push_back(matrix[i][right]);
                --right;
            }
            // bottom line
            if (up <= down && left <= right) {
                for (int i = right; i >= left; --i)
                    res.push_back(matrix[down][i]);
                --down;
            }
            // left line
            if (up <= down && left <= right) {
                for (int i = down; i >= up; --i)
                    res.push_back(matrix[i][left]);
                ++left;
            }
        }
        return res;
    }
};