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.
Input: "abbxxxxzzy"
Output: [[3,6]]
Explanation: "xxxx" is the single large group with starting 3 and ending positions 6.Input: "abc"
Output: []
Explanation: We have "a","b" and "c" but no large group.Input: 'abcdddeeeeaabbbcd'
Output: [[3, 5], [6, 9], [12, 14]]