reverse-a-string.java

public class Solution {
  public String reverseString(String s) {
    if (s == null) {
      return null;
    }
    int len = s.length();
    char[] cs = new char[len];
    for (int i = 0; i < len; i++) {
      cs[len - 1 - i] = s.charAt(i);            
    }
    return new String(cs);
  }
}