Write a function that takes a string as input and returns the string reversed.

Example:
Given s = “hello”, return “olleh”.

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public class Solution {
    public String reverseString(String s) {
        if (s == null) return null;
        StringBuilder sb = new StringBuilder();
        
        for (int i = s.length() - 1; i >= 0; i--) {
            sb.append(s.charAt(i));
        }
        
        return sb.toString();
    }
}