在一个由小写字母构成的字符串 S 中，包含由一些连续的相同字符所构成的分组。
例如，在字符串 S = “abbxxxxzyy” 中，就含有 “a”, “bb”, “xxxx”, “z” 和 "yy"这样的一些分组。我们称所有包含大于或等于三个连续字符的分组为较大分组。找到每一个较大分组的起始和终止位置。最终结果按照字典顺序输出。

样例
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]]
注意事项
1 <= S.length <= 1000

class Solution {
public:
    /**
     * @param S: a string
     * @return: the starting and ending positions of every large group
     */
    vector<vector<int>> largeGroupPositions(string &S) {
        // Write your code here
        int cnt=1;
        char c=S[0];
        vector<vector<int>> result;
        for (int i = 1; i < S.size(); i++) {
            /* code */
            if(cnt>=3&&c!=S[i])
            {
                vector<int> temp={i-cnt,i-1};
                result.push_back(temp);
                c=S[i];
                cnt=1;
            }
            else if(c!=S[i]) {c=S[i];cnt=1;}
            else if(c==S[i])cnt++;
        }
        if(cnt>=3)
        {
            vector<int>temp={S.size()-cnt,S.size()-1};
            result.push_back(temp);
        }
        return result;
    }
};