Leetcode: Spiral Matrix II @Python

class Solution:
    # @return a list of lists of integer
    def generateMatrix(self, n):
        rst=[[0 for i in range(n)] for j in range(n)]
        up=0
        down=n-1
        left=0
        right=n-1
        elem=1
        while up<=down and left<=right:
            for i in range(left,right+1):
                rst[up][i]=elem
                elem+=1
            up+=1
            if up<=down:
                for i in range(up,down+1): 
                    rst[i][right]=elem
                    elem+=1
                right-=1
            else:
                break
            if left<=right:
                for i in reversed(range(left,right+1)): 
                    rst[down][i]=elem
                    elem+=1
                down-=1
            else:
                break
            if down>=up:
                for i in reversed(range(up,down+1)): 
                    rst[i][left]=elem
                    elem+=1
                left+=1
            else:
                break
        return rst