I'm getting ready for an interview and practicing time complexity and I'm ok at recognizing the complexity for the code I've written myself but have a harder time when I throw in methods from the java library.

For example, I read that the method concat was similar to += and that concat time complexity was O(n^2). So if I reverse a string with the code below does that mean my time complexity is O(n^2)? With a space complexity of O(n) since? Where n is the length of the string.

String str="hello"
String s="";
for(int i =str.length()-1; i>=0;i--){
s+=str.substring(i,i+1);
}

With this code is the space complexity O(n) and the time complexity O(n)? Where n is the length of the string.

String str="hello";
char[] cArr = str.toCharArray();
int i =0;
int j = cArr.length-1;
char temp;
while(i<j){
    temp =cArr[i];
    cArr[i]=cArr[j];
    cArr[j]=temp;
    i++;
    j--;
}
str=String.valueOf(cArr);