class Solution {
public:
    int removeDuplicates(vector<int>& nums) {
        // fast and slow start from 0
        // fast go first
        // if nums[fast] != nums[slow], slow++ and swap
        // return slow
        int fast = 1, slow = 0;

        // 0 1 0 1 1 2 2 3 3 4
        // 0 1 2 1 1 0 2 3 3 4
        //     s       f
        for(; fast < nums.size(); fast++) {
            if(nums[fast] != nums[slow]) {
                slow++;
                nums[slow] = nums[fast];
            }
        }

        return slow + 1;
    }
};