Positions of Large Groups

In a string S of lowercase letters, these letters form consecutive groups of the same character.

For example, a string like S = "abbxxxxzyy" has the groups "a", "bb", "xxxx", "z" and "yy".

Call a group large if it has 3 or more characters.  We would like the starting and ending positions of every large group.

The final answer should be in lexicographic order.

Example 1:

Input: "abbxxxxzzy"
Output: [[3,6]]
Explanation: "xxxx" is the single large group with starting  3 and ending positions 6.

Example 2:

Input: "abc"
Output: []
Explanation: We have "a","b" and "c" but no large group.

Example 3:

Input: "abcdddeeeeaabbbcd"
Output: [[3,5],[6,9],[12,14]]

Note:  1 <= S.length <= 1000

Solution: Keep a variable to avoid boundary checking. 

class Solution {
    public List<List<Integer>> largeGroupPositions(String S) {
        int left = 0, right = 0, n = S.length();
        char current = 'A';
        
        List<List<Integer> > result = new ArrayList<List<Integer> >();
        while (right < n) {
            while (right < n && S.charAt(right) == current) {
                right++;
            }
            
            if (right - left >= 3) {
                List<Integer> temp = new ArrayList<>();
                temp.add(left);
                temp.add(right - 1);
                result.add(temp);
            }
            
            left = right;
            right = left;
            if (left < n) {
                current = S.charAt(left);
            }
        }
        
        return result;
    }
}