830. 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

 

//Time: O(n), Space: O(1)
    public List<List<Integer>> largeGroupPositions(String S) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        
        if (S == null || S.length() == 0) {
            return result;
        }
        
        int j = 0;
        
        for (int i = 0; i <= S.length(); i++) {
            if (i == S.length() || S.charAt(i) != S.charAt(j)) {//two pointer题目一定不要忘记最后一位 eg：“aaa”
                if (i - j >= 3) {
                    List<Integer> temp = new ArrayList<Integer>();
                    temp.add(j);
                    temp.add(i - 1);
                    result.add(temp);
                }
                
                j = i;
            }
        }
        
        return result;
    }