<分区>
我正在尝试使用递归返回字符串的反转，但出现了一些错误。我是递归的新手，所以我真的不知道从哪里开始。我得到的错误是:
Exception in thread "main" java.lang.NullPointerException
at java.lang.String.concat(String.java:2027)
at Recursion.reverseString(Recursion.java:30)
at Recursion.reverseString(Recursion.java:30)
at Recursion.reverseString(Recursion.java:30)
at Recursion.reverseString(Recursion.java:30)
at Recursion.reverseString(Recursion.java:30)
at Recursion.main(Recursion.java:46)
这是我的代码:
public static String reverseString (String inString) {
String result = "";
if (inString.length() > 0) {
// if the string is empty
result = inString.charAt(inString.length()-1) + "";
result.concat(reverseString(inString.substring(0, inString.length()-1)));
return result;
} else {
return null;
}
}
// the testers
public static void main(String[] args){
String inString = "abcde";
// test the reverseString
String revString = reverseString(inString);
System.out.println(revString);
}
}