Positions of Large Groups

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".

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

Solution

class Solution {
public:
    vector<vector<int>> largeGroupPositions(string S) {
        vector<vector<int>> ans;
        int n = S.size();
        for(int i = 0; i < n; ){
            int j = i + 1;
            while(j < n && S[i] == S[j]) ++j;
            if(j - i >= 3)
                ans.push_back({i, j - 1});
            i = j;
        }
        return ans;
    }
};