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# 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

```cpp
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;
    }
};
```