Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.
Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.
For the purpose of this problem, we define empty string as valid palindrome.
判断给出的字符串是否是回文
自己的思路如下，正则表达式格式化给出的字符串，使用StringBuilder.reverse()方法得到反转的字符串，如果与格式化后的字符串一致，那么返回true，否则返回false
public boolean isPalindrome(String s) {
if(s == null){
return false;
}
s = s.replaceAll("\\W+", "").toLowerCase();
if(s.isEmpty()){
return true;
}
return s.equals(new StringBuilder(s).reverse().toString());
}
这种用到了java本身提供的特性，还有一种办法是首指针++,尾指针--,并判断s[i]==s[j]
public boolean isPalindrome(String s) {
if (s == null) {
return false;
}
s = s.toLowerCase();
for(int i = 0, j = s.length() - 1; i < j;){
if(!Character.isLetterOrDigit(s.charAt(i))){
i++;
}else if(!Character.isLetterOrDigit(s.charAt(j))){
j--;
}else{
if(s.charAt(i++) != s.charAt(j--)){
return false;
}
}
}
return true;
}
