59. 螺旋矩阵 II

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        vector<vector<int>> ans(n);
        for (int i = 0; i < ans.size(); i++)   //注意先设定好ans的size
            ans[i].resize(n);
        int num = n * n;
        int i = 1;
        int l = 0;
        int r = n - 1;
        int u = 0;
        int d = n - 1;
        while (i <= num) {
            for (int j = l; j <= r && i <= num; i++, j++)
                ans[u][j] = i;
            u++;
            for (int j = u; j <= d && i <= num; i++, j++)
                ans[j][r] = i;
            r--;
            for (int j = r; j >= l && i <= num; i++, j--)
                ans[d][j] = i;
            d--;
            for (int j = d; j >= u && i <= num; i++, j--)
                ans[j][l] = i;
            l++;
        }
        return ans;
    }
};