[英]String palindrome checker in java not working?
I am trying to write a program for these instructions: 我正在尝试为这些指令编写程序:
Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
给定一个字符串,请仅考虑字母数字字符并忽略大小写,从而确定它是否是回文。 Note: For the purpose of this problem, we define empty
String
as valid palindrome.注意:针对此问题,我们将空
String
定义为有效回文。 https://leetcode.com/problems/valid-palindrome/https://leetcode.com/problems/valid-palindrome/
For some reason, the .reverse()
in the last line doesn't reverse it. 由于某种原因,最后一行中的
.reverse()
不会将其反转。 I've tried debugging by adding print statements and I see that the string DOES reverse earlier on. 我已经尝试通过添加打印语句进行调试,并且发现字符串确实在更早的时候反转了。 What is going on here?
这里发生了什么? Please guide me!
请指导我!
public static boolean isPalindrome(String s) {
StringBuilder sb = new StringBuilder();
for (int i = 0; i < s.length(); i++) {
if (Character.isLetter(s.charAt(i))) {
sb.append(s.charAt(i));
}
}
String x = sb.reverse().toString();
System.out.println(x);
if (sb.length() == 0)
return true;
else
return sb.toString().equals(x);
}
The problem is that reverse()
changes the StringBuilder
itself. 问题是
reverse()
更改StringBuilder
本身。 So you are comparing the reverse, with the reverse again. 因此,您正在比较反向,再次与反向。 Instead change your method to:
而是将方法更改为:
String x = sb.toString();
return sb.reverse().toString().equals(x);
Which will compare the StringBuilder
before reversing, with the reversed StringBuilder
. 它将在反转之前将
StringBuilder
与反转的StringBuilder
。 Note that the if(sb.length == 0)
is not necessary, as if it is empty, sb.reverse().toString().equals(x)
will still return true. 请注意,
if(sb.length == 0)
不是必需的,就好像它是空的一样, sb.reverse().toString().equals(x)
仍将返回true。
Also your search is case sensitive, when the problem statement says that it should match regardless of case. 当问题陈述指出无论大小写均应匹配时,搜索也区分大小写。 Change where you append to:
更改附加到的位置:
if (Character.isLetter(s.charAt(i))) {
sb.append(Character.toLowerCase(s.charAt(i)));
}
Also you can take advantage of replaceAll
and toLowerCase()
to shorten your method to: 您还可以利用
replaceAll
和toLowerCase()
将方法缩短为:
public static boolean pali(String s) {
String copy = s.toLowerCase().replaceAll("[^a-z]", "");
return new StringBuilder(copy).reverse().toString().equals(copy);
}
因为在第一次调用sb.reverse()
sb更改了它的状态。
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