To lower Case

这题不多说，不会做的闭门思过。

Implement function ToLowerCase() that has a string parameter str, and returns the same string in lowercase.

 

Example 1:

Input: "Hello"
Output: "hello"
Example 2:

Input: "here"
Output: "here"
Example 3:

Input: "LOVELY"
Output: "lovely"

std::string res = str;
        for (std::size_t i = 0; i < str.size(); i++) {
            if (str[i] >= 'A' && str[i] <= 'Z') {
                res[i] = 'a' + str[i] - 'A';
            }
        }
        return res;