Leetcode97: Spiral Matrix II

Given an integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.

For example,
Given n = 3,

You should return the following matrix:

[
 [ 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));
        int beginx = 0;
        int beginy = 0;
        int endx = n-1;
        int endy = n-1;
        int a = 1;
        while(true)
        {
            for(int i = beginx; i <= endx; i++)
            {
                res[beginy][i] = a++;
            }
            if(++beginy > endy) break;
            for(int i = beginy; i <= endy; i++)
            {
                res[i][endx] = a++;
            }
            if(beginx > --endx) break;
            for(int i = endx; i >= beginx; i--)
            {
                res[endy][i] = a++;
            }
            if(beginy > --endy) break;
            for(int i = endy; i >= beginy; i--)
            {
                res[i][beginx] = a++;
            }
            if(++beginx > endx) break;
        }
        return res;
    }
};