leetcode 709. To Lower Case

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"

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