830. Positions of Large Groups

1. Description

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”.
A group is identified by an interval [start, end], where start and end denote the start and end indices (inclusive) of the group. In the above example, “xxxx” has the interval [3,6].
A group is considered large if it has 3 or more characters.
Return the intervals of every large group sorted in increasing order by start index.

2. Example

Example 1:
Input: s = “abbxxxxzzy”
Output: [[3,6]]
Explanation: “xxxx” is the only large group with start index 3 and end index 6.

Example 2:
Input: s = “abc”
Output: []
Explanation: We have groups “a”, “b”, and “c”, none of which are large groups.

Example 3:
Input: s = “abcdddeeeeaabbbcd”
Output: [[3,5],[6,9],[12,14]]
Explanation: The large groups are “ddd”, “eeee”, and “bbb”.

Example 4:
Input: s = “aba”
Output: []

3. Constraints

• 1 <= s.length <= 1000
• s contains lower-case English letters only.

4. Solutions

My Accepted Solution

n = m_str.size()
Time complexity: O(n)
Space complexity: O(n)

class Solution 
{
public:
    // vector<vector<int>> largeGroupPositions(string s)
    vector<vector<int>> largeGroupPositions(string &m_str) 
    {
        vector<vector<int>> result;
        m_str.push_back('*'); // guard
        for(int left = 0, right = 0; left < m_str.size(); left = right)
        {
            while(right < m_str.size() && m_str[right] == m_str[left]) right++;
            
            if(right - left >= 3)
                result.push_back({left, right - 1});
        }
        
        return result;
    }
};