使用0,1,2,3四个值表示方向以便切换, 用maxUp, maxDown, maxLeft, maxRight四个变量记录四个边界。当 curr >= n * n 就结束。

0,1,2,3 four values are used to indicates direction, maxUp, maxDown, maxLeft, maxRight four variables are used to record four boundaries. When curr >= n * n program can return.

class Solution:
    # @return a list of lists of integer
    def generateMatrix(self, n):
        direct = 0 # 0 go right, 1 go down, 2 go left, 3 go up
        maxUp = maxLeft = 0
        maxDown = maxRight = n - 1
        A = [ [0 for j in xrange(n)] for i in xrange(n) ]
        curr = 0
        while True:
            if direct == 0: # go right
                for i in xrange(maxLeft, maxRight + 1): curr += 1; A[maxUp][i] = curr
                maxUp += 1
            elif direct == 1: # go down
                for i in xrange(maxUp, maxDown + 1): curr += 1; A[i][maxRight] = curr
                maxRight -= 1
            elif direct == 2: # go left
                for i in reversed(xrange(maxLeft, maxRight + 1)): curr += 1; A[maxDown][i] = curr
                maxDown -= 1
            else: # go up
                for i in reversed(xrange(maxUp, maxDown + 1)): curr += 1; A[i][maxLeft] = curr
                maxLeft += 1
            if curr >= n * n: return A
            direct = (direct + 1) % 4