2-5

class Solution {
    public int removeDuplicates(int[] nums) {
        // [1, 1, 1, 1, 2, 2, 3, 4, 4, 4]
        //  n  d  d  d  n  d  n  n  d  d
        // n – non-duplicate.
        // d – duplicate.

        // We will put non-duplicated numbers at positions
        // 0, 1, 2, 3, 4, ..., etc. one-by-one.

        // Number nums[0] should stay at position 0.
        // We will start filling non-duplicated numbers
        // from position 1.
        int j = 1;

        // Iterate starting from i = 1.
        for (int i = 1; i < nums.length; i++) {
            // If it's a duplicate (d), skip it.
            if (nums[i] == nums[i - 1]) {
                continue;
            }

            // Otherwise (n), put it at index j,
            // and then increment j.
            nums[j++] = nums[i];
        }
        return j;
    }
}