leetcode#709. 转换成小写字母

class Solution {
public:
    string toLowerCase(string str) 
    {
        for(int i = 0; i < str.size(); ++i)
        {
            if(str[i] >= 'A' && str[i] <= 'Z')
                str[i] += ('a' - 'A');
        }
        return str;
    }
};