출처 : https://leetcode.com/problems/to-lower-case/?envType=study-plan-v2&envId=programming-skills
Given a string s, return the string after replacing every uppercase letter with the same lowercase letter.

class Solution {
public String toLowerCase(String s) {
String res = "";
for (int i = 0; i < s.length(); i++) {
if (Character.isUpperCase(s.charAt(i))) {
res += Character.toLowerCase(s.charAt(i));
} else {
res += s.charAt(i);
}
}
return res;
}
}