spiral matrix ii

class (object):
    def generateMatrix(self, n):
        """
        :type n: int
        :rtype: List[List[int]]
        """
        if n == 0:
            return []
        
        res = [[0 for _ in range(n)] for _ in range(n)]
        
        tr = lc = 0
        br = rc = n - 1
        num = 1
        while num <= n**2:
            if num <= n**2:
                # top 
                for i in range(lc, rc+1):
                    res[tr][i] = num
                    num += 1
                tr += 1
            if num <= n**2:
                # right
                for i in range(tr, br+1):
                    res[i][rc] = num
                    num += 1
                rc -= 1
            if num <= n**2:
                # bot
                for i in range(rc, lc-1, -1):
                    res[br][i] = num
                    num += 1
                br -= 1
            if num <= n**2:
                # left
                for i in range(br, tr-1, -1):
                    res[i][lc] = num
                    num += 1
                lc += 1
            
        return res