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 of nums being 1, 1, 2, 2 and 3. It doesn’t matter what you leave beyond the new length.

双指针问题：

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