[LeetCode] 830. position of large groups

• Problem: position of large groups, 計算連續字母的開始與結束位置
• Solution: 
• 直接 iterate 整個字串
• 第一個字母直接紀錄, 最後一組 group 要特別處理

class Solution {
public:
    vector> collectLargeGroups(const char *str, int len) {
        char currCh;
        int length = 0, start = 0, end = 0;
        vector> result;
        const int LARGE_SIZE = 3;
        
        if (NULL == str) return result;
        currCh = str[0];
        length = 1;
        
        for(size_t i = 1; i < len; ++i)
        {
            if (currCh != str[i])
            {
                end = i - 1;
                length = end - start + 1;

                if (length >= LARGE_SIZE)
                {
                    vector group{start, end};
                    result.push_back(group);
                }
                start = i;
                currCh = str[i];
            }
        }
        
        // add last group
        end = len - 1;
        length = end - start + 1;
        if (length >= LARGE_SIZE)
        {
            vector group{start, end};
            result.push_back(group);
        }
        
        return result;
    }
    
    vector> largeGroupPositions(string S) {
        return collectLargeGroups(S.c_str(), S.length());
    }
};