Remove Duplicates from Sorted Array II

Given a sorted array, remove the duplicates in place such that each element can appear atmost twice and return the new length.

Do not allocate extra space for another array, you must do this in place with constant memory.

For example,
Given input array A = [1,1,1,2],

Your function should return length = 3, and A is now [1,1,2].

Method:

We set the index to new value if current value does not equal to number in (size - 2) index.

Solution:

Time: O(n)
Space: O(1)

public class Solution {
    public int removeDuplicates(ArrayList<Integer> a) {
        if (a.size() <= 2) return a.size();
        int size = 2;
        for (int i = 2; i < a.size(); i ++) {
            int curr = a.get(i);
            int prev = a.get(size - 2);
            if (curr != prev) {
                a.set(size, curr);
                size ++;
            }
        }
        while (a.size() > size) {
            a.remove(a.size() - 1);
        }
        return size;
    }
}