830. Positions of Large Groups

1，题目要求
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.

对于一个给定的字符串，找出其中连续出现次数超过三次的子字符串，记录这个子字符串的开始和结束的索引位置，返回这样一个索引对序列。

2，题目思路
没有太多难点，用直接法来做是最快的。值得注意的是，直接（straight-forward）并不等同于暴力（brute）。暴力是在牺牲大量时间和空间基础之上的求解办法，而好的直接法是从问题的本质直接入手的办法，在性能上未必没有间接法要好。
对于这一题，只需要遍历整个字符串，并以此记录相邻字母出现的次数以及该字母的起始位置。当其出现次数大于等于3时，直接构造一个索引对并添加到最终解中即可。

3，程序源码

class Solution {
public:
    vector<vector<int>> largeGroupPositions(string S) {
        int count = 1,pos = 0;
        vector<vector<int>> res;
        for(int i = 0;i<S.size();i++)
        {
            vector<int> tmp (2,0);
            if(S[i] == S[i+1])
                count ++;
            else
            {
                if(count>=3)
                {
                    tmp[0] = pos;
                    tmp[1] = pos+ count - 1;
                    res.push_back(tmp);
                }
                pos = i+1;
                count = 1;
            }
        }
        return res;
    }
};