Leetcode59. Spiral Matrix II螺旋矩阵2

给定一个正整数 n，生成一个包含 1 到 n2 所有元素，且元素按顺时针顺序螺旋排列的正方形矩阵。

示例:

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

class Solution {
public:
    vector<vector<int> > generateMatrix(int n)
    {
        vector<vector<int> > res(n, vector<int>(n, 0));
        int left = 0;
        int up = 0;
        int right = n - 1;
        int down = n - 1;
        int cnt = 1;
        while(up <= down && left <= right)
        {
            for(int i = left; i <= right; i++)
                res[up][i] = cnt++;
            up++;
            if(up > down)
                break;
            for(int i = up; i <= down; i++)
                res[i][right] = cnt++;
            right--;
            if(left > right)
                break;
            for(int i = right; i >= left; i--)
                res[down][i] = cnt++;
            down--;
            if(up > down)
                break;
            for(int i = down; i >= up; i--)
                res[i][left] = cnt++;
            left++;
            if(left > right)
                break;
        }
        return res;
    }
};