Solution.py

"""
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
"""
class Solution(object):
    def largeGroupPositions(self, S):
        """
        :type S: str
        :rtype: List[List[int]]
        """
        if not S:
            return []
        ret = []
        p, n = S[0], 1
        for i, c in enumerate(S):
            if i == 0:
                continue
            if c == p:
                n += 1
            else:
                if n >= 3:
                    ret.append([i - n, i - 1])
                n = 1
            p = c
        if n >= 3:
            ret.append([i - n + 1, i])
        return ret