leetcode-830-Positions of Large Groups

题目描述：

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

要完成的函数：

vector<vector<int>> largeGroupPositions(string S)

说明：

1、给定一个字符串S，如果一个字符连续出现三次及三次以上，那么它就是一个“大组合”，要求找出所有“大组合”的起始位置和结束位置，最终以vector<vector<int>>的形式返回。

2、明白题意，这又是一道简单题。

直接贴上代码（附详解），如下：

    vector<vector<int>> largeGroupPositions(string S)
    {
        int s1=S.size(),i=0,j;
        vector<vector<int>>res;//最后返回的vector
        vector<int>res1;//子vector
        while(i<s1-2)
        {
            if(S[i]==S[i+1]&&S[i]==S[i+2])//如果满足条件
            {
                res1.clear();//清空res1
                res1.push_back(i);//插入起始位置到res1
                j=i+3;
                while(j<s1)//找到不等于S[i]的字符位置
                {
                    if(S[i]==S[j])
                        j++;
                    else
                        break;
                }
                res1.push_back(j-1);//插入结束位置到res1
                res.push_back(res1);//res1插入到res里面
                i=j;//更新i的值
            }
            else
                i++;
        }
        return res;
    }

上述代码实测13ms，因为服务器接收到的cpp submissions有限，所以没有打败的百分比。