思路：模拟，给出上下左右4个方向的边界，逐步收缩

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        int left = 0;
        int right = n-1;
        int top = 0;
        int bottom = n-1;
        int k = 1;
        vector<vector<int>> ans (n,vector<int>(n));
        while(k <= n*n){
            for(int i = left;i<=right;++i,++k)  ans[top][i] = k;
            top++;
            for(int i = top;i<=bottom;++i,++k)  ans[i][right] = k;
            right--;
            for(int i = right;i>=left;--i,++k)  ans[bottom][i] = k;
            bottom--;
            for(int i = bottom;i>=top;--i,++k)  ans[i][left] = k;
            left++;
        }
        return ans;
    }
};