class Solution {
public:
    int removeDuplicates(vector<int>& nums) {
        int n = nums.size();
        if(n <= 2) return n;
        int i = 2;
        for(int j = 2; j < n; j ++)
            if(nums[j] != nums[i - 2]){
                nums[i] = nums[j];
                i ++; //如果不等于，i和j是同时加的，即下一轮ij相等
                        //如果等的话，略过j这个元素
            } 
        return i;

    }
};