830. Positions of Large Groups

1. Question:

url https://leetcode.com/problems/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

2. Solution:

class Solution:
    def largeGroupPositions(self, S):
        """
        :type S: str
        :rtype: List[List[int]]
        """
        if S is None or len(S) < 3:
            return []

        re_list = []
        bf_value = S[0]
        start = 0
        end = 0
        for i in range(1, len(S)):
            if S[i] == S[i - 1]:
                continue
            end = i - 1
            if end - start + 1 >= 3:
                re_list.append([start, end])
            start = i
        if len(S) - start >= 3:
            re_list.append([start, len(S) - 1])

        return re_list

3. Complexity Analysis

Time Complexity : O(N)

Space Complexity: O(1)