Remove Duplicates from sorted array

//code1:
O(n),O(1)
class Solution{
    public:
        int removeDuplicates(vector<int>& nums){
            if(nums.empty()) return 0;
            
            int index = 0;
            for(int i = 1; i < nums.size(); ++i){
                if(nums[index] != nums[i])
                    nums[++index] = nums[i];
            }
            
            return index + 1;
        }
};


//code2:
class Solution{
    public:
        int removeDuplicates(vector<int>& nums){
            return distance(nums.begin(),unique(nums.begin(),nums.end()));
        }
};

//code3:
class Solution{
    public:
        int removeDuplicates(vector<int>& nums){
            return distance(nums.begin(),removeDuplicates(nums.begin(),nums.end(),nums.begin()));
        }
    
    template<typename InIt, typename OutIt>
    OutIt removeDuplicates(InIt first,InIt last, OutIt output){
        while(first != last){
            *output++ = *first;
            first = upper_bound(first,last,*first);
        }
        return output;
    }
};