原题是：
Given a non-negative integer represented as a non-empty array of digits, plus one to the integer.
You may assume the integer do not contain any leading zero, except the number 0 itself.
The digits are stored such that the most significant digit is at the head of the list.
思路是：
通过Python中Index-1，-2，-3,来从“个位”数，模拟加法的过程。
逻辑划分是：
非首位数，分成9和不是9来处理；首位数，分成9和非9来处理。
代码是：
class Solution:
def plusOne(self, digits):
"""
:type digits: List[int]
:rtype: List[int]
"""
i = -1
while i > - len(digits) :
if digits[i] < 9 :
digits[i] += 1
return digits
elif digits[i] == 9:
digits[i] = 0
i -= 1
if digits[0] == 9:
digits[0] = 0
digits.insert(0,1)
else:
digits[0] += 1
return digits
