Remove Duplicates from Sorted Array II

Follow up for "Remove Duplicates":
What if duplicates are allowed at most twice?

For example,
Given sorted array nums = [1,1,1,2,2,3],

Your function should return length = 5, with the first five elements ofnums being 1, 1, 2, 2 and3. It doesn't matter what you leave beyond the new length.

Solution:

class Solution {public:    int removeDuplicates(vector<int>& nums) {        int len = nums.size();        if(len == 0) return 0;        if(len == 1) return 1;        int sum = 1, pre = nums[1];        for(int i = 2; i < nums.size(); ++i)        {            if(nums[i] != nums[i-2])            {                nums[sum++] = pre;                pre = nums[i];            }        }        nums[sum++] = pre;        return sum;    }};