3-4

class Solution {
    public int removeDuplicates(int[] nums) {
        // Numbers nums[0] and nums[1] will stay where they are.
        int insertPosition = 2;

        // We will iterate over numbers starting from i = 2,
        // and numbers that we need at insertPosition one-by-one.
        for (int i = 2; i < nums.length; i++) {
            if (nums[i] != nums[insertPosition - 2]) {
                nums[insertPosition++] = nums[i];
            }
        }
        return insertPosition;

        //                           0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
        //                   nums = [1, 1, 1, 1, 1, 2, 2, 2, 2, 2,  3,  3,  3].
        // insertPosition = 2              ^
        //              i = 2              ^
        // nums[i] == nums[insertPosition - 2], so skip this 1 as it was included twice.

        //                           0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
        //                   nums = [1, 1, 1, 1, 1, 2, 2, 2, 2, 2,  3,  3,  3].
        // insertPosition = 2              ^
        //              i = 3                 ^
        // nums[i] == nums[insertPosition - 2], so skip this 1 as it was included twice.

        //                           0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
        //                   nums = [1, 1, 1, 1, 1, 2, 2, 2, 2, 2,  3,  3,  3].
        // insertPosition = 2              ^
        //              i = 4                    ^
        // nums[i] == nums[insertPosition - 2], so skip this 1 as it was included twice.

        //                           0, 1,  2,  3, 4, 5, 6, 7, 8, 9, 10, 11, 12
        //                   nums = [1, 1, *2*, 1, 1, 2, 2, 2, 2, 2,  3,  3,  3].
        // insertPosition = 2               ^
        //              i = 5                         ^
        // nums[i] != nums[insertPosition - 2], so put 2 at insertPosition = 2
        // and increment insertPosition by 1.

        //                           0, 1, 2,  3,  4, 5, 6, 7, 8, 9, 10, 11, 12
        //                   nums = [1, 1, 2, *2*, 1, 2, 2, 2, 2, 2,  3,  3,  3].
        // insertPosition = 3                  ^
        //              i = 6                            ^
        // nums[i] != nums[insertPosition - 2], so put 2 at insertPosition = 3
        // and increment insertPosition by 1.

        //                           0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
        //                   nums = [1, 1, 2, 2, 1, 2, 2, 2, 2, 2,  3,  3,  3].
        // insertPosition = 4                    ^
        //              i = 7                             ^
        // nums[i] != nums[insertPosition - 2], so skip this 2 as it was included twice.

        //                           0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
        //                   nums = [1, 1, 2, 2, 1, 2, 2, 2, 2, 2,  3,  3,  3].
        // insertPosition = 4                    ^
        //              i = 8                                ^
        // nums[i] != nums[insertPosition - 2], so skip this 2 as it was included twice.

        //                           0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
        //                   nums = [1, 1, 2, 2, 1, 2, 2, 2, 2, 2,  3,  3,  3].
        // insertPosition = 4                    ^
        //              i = 8                                ^
        // nums[i] != nums[insertPosition - 2], so skip this 2 as it was included twice.

        //                           0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
        //                   nums = [1, 1, 2, 2, 1, 2, 2, 2, 2, 2,  3,  3,  3].
        // insertPosition = 4                    ^
        //              i = 9                                   ^
        // nums[i] != nums[insertPosition - 2], so skip this 2 as it was included twice.

        //                           0, 1, 2, 3,  4,  5, 6, 7, 8, 9, 10, 11, 12
        //                   nums = [1, 1, 2, 2, *3*, 2, 2, 2, 2, 2,  3,  3,  3].
        // insertPosition =  4                    ^
        //              i = 10                                        ^
        // nums[i] != nums[insertPosition - 2], so put 3 at insertPosition = 4
        // and increment insertPosition by 1.

        //                           0, 1, 2, 3, 4,  5,  6, 7, 8, 9, 10, 11, 12
        //                   nums = [1, 1, 2, 2, 3, *3*, 2, 2, 2, 2,  3,  3,  3].
        // insertPosition =  5                       ^
        //              i = 11                                            ^
        // nums[i] != nums[insertPosition - 2], so put 3 at insertPosition = 5
        // and increment insertPosition by 1.

        //                           0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
        //                   nums = [1, 1, 2, 2, 3, 3, 2, 2, 2, 2,  3,  3,  3].
        // insertPosition =  6                         ^
        //              i = 12                                              ^
        // nums[i] != nums[insertPosition - 2], so skip this 3 as it was included twice.

        // Return insertPosition = 6, as six values at indices 0..5 are the answer.
        // All values after position 6 shall be ignored like this:
        //                           0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
        //                   nums = [1, 1, 2, 2, 3, 3, _, _, _, _,  _,  _,  _].
    }
}