螺旋矩阵ii

定义左边界右边界（由矩阵的性质可得，左边界也是上边界、右边界也是下边界），以左边界作为循环条件，循环打印即可：

//
// Created by jt on 2020/9/29.
//
#include <vector>
using namespace std;

class Solution {
public:
    /**
     *
     * @param n int整型
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > generateMatrix(int n) {
        // write code here
        vector<vector<int>> res(n, vector<int>(n));
        for (int left = 0, right = n - 1, current = 1; left <= n / 2; ++left, --right) {
            // 上
            for (int j = left; j <= right; ++j) {
                res[left][j] = current++;
            }
            // 右
            for (int j = left + 1; j <= right; ++j) {
                res[j][right] = current++;
            }
            // 下
            for (int j = right-1; j >= left; --j) {
                res[right][j] = current++;
            }
            // 左
            for (int j = right-1; j > left; --j) {
                res[j][left] = current++;
            }
        }
        return res;
    }
};