螺旋矩阵II(leetcode 59)

转圈过程（边界处理）遵循循环不变量的原则，坚持一个原则处理每一条边，左闭右开处理

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        vector<vector<int>> num(n, vector<int>(n, 0));
        int startx = 0;
        int starty = 0;
        int offset = 1;
        int count = 1;
        int loop = n/2;
        int i;
        int j;
        while(loop --)
        {
            
            for( j = starty; j < n - offset; j++)
            {
                num[startx][j] = count;
                count ++;
            }
            for( i = startx; i < n - offset; i++)
            {
                num[i][j] = count;
                count ++;
            }
            for( ; j > starty; j--)
            {
                num[i][j] = count;
                count ++;
            }
            for( ; i > startx; i--)
            {
                num[i][j] = count;
                count ++;
            }
            startx++;
            starty++;
            offset++;
        }
        if(n % 2)
        {
            num[n/2][n/2] = count;
        }
        return num;
    }
};