【LeetCode每天一题】Spiral Matrix II(螺旋数组II)

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

Example:

Input: 3
Output:
[
 [ 1, 2, 3 ],
 [ 8, 9, 4 ],
 [ 7, 6, 5 ]
]
思路

---

   之前做过一个旋转数组的题，当时是旋转打印矩阵，这次只是构造一个旋转数组。方案还是一样的，条件变量也一样。时间复杂度为O(n*n),  空间复杂度为O(n)。
解决代码

---

     def generateMatrix(self, n):
         """
         :type n: int
         :rtype: List[List[int]]
         """
         if n == 0 or n == 1:
             return 0 if n == 0 else [[1]]
         nums = 1
         up, down, left, right = 0, n-1, 0, n-1   # 上下左右变量
         res = []
         for i in range(n):       # 结果矩阵
             res.append([0]*n)

         while left < right:     # 循环条件，也可以是 up< down。 因为是n*n矩阵，所以可以这样。
             for i in range(left, right):
                 res[up][i] = nums
                 nums+= 1

             for i in range(up, down):
                 res[i][right] = nums
                 nums += 1

             for i in range(right, left, -1):
                 res[down][i] = nums
                 nums += 1

             for i in range(down, up, -1):
                 res[i][left] = nums
                 nums += 1
             up, down, left, right = up+1, down-1, left+1, right-1

         if n % 2:                 # 如果是奇数的话，中心还有一个元素。
             res[left][right] = nums
         return res