This problem is easy to solve by double pointers, like below:
class Solution(object):
def removeDuplicates(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
if len(nums) == 0:
return 0
pre = 0
for i in range(len(nums)):
if nums[i] != nums[pre]:
pre += 1
nums[pre] = nums[i]
return pre + 1