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; } };
