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]]

较大分组的位置

关键位置在前后元素不同时的处理,索引需要重置,并判断重复次数.字符串末尾增加一个#字符是为了处理末尾满足重复次数大于三的情况.

class Solution(object):
    def largeGroupPositions(self, S):
        """
        :type S: str
        :rtype: List[List[int]]
        """
        last_item = None
        left_index = -1
        nums_list = []
        for i, item in enumerate(S + '#'):
            if last_item != item:
                if i - left_index > 2:
                    nums_list.append([left_index, i - 1])
                left_index = i
            last_item = item
        return nums_list