119. 杨辉三角 II

https://leetcode-cn.com/problems/pascals-triangle-ii/

class Solution:
    def getRow(self, rowIndex):
        """
        :type rowIndex: int
        :rtype: List[int]
        """
        
        res = [0] * (rowIndex+1)
        num, j = 1, 1
        res[0] = 1
        if rowIndex == 0:
            return res
        #用组合数C（rowIndex，k），k从1到rowIndex的一半，剩下一半对称
        mid =  rowIndex // 2
        for i in range(1, mid+1):
            num *= (rowIndex + 1 - i)
            num //= j
            j += 1
            res[i] = num
 
        if rowIndex % 2 == 0:
            res[mid+1:] = res[mid-1::-1]
        else:
            res[mid+1:] = res[mid::-1]
        return res