**Algorithm**

The solution is straight-forward. We check each of the char of the String. Then concat every single char to string

String reverseString(String s) {

if (s == null || s.length() == 0)

return "";

StringBuilder ret = new StringBuilder();

for (int i = s.length() - 1; i > -1; i --) {

ret.append(s.charAt(i));

}

return ret.toString();

}

**Complexity Analysis**

The run time depends on the String in s. the time complexity is O(1)O(1).