[LeetCode]Valid Palindrome

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.

双指针，一个从左往右，一个从右往左，相遇比较

public class Solution {public boolean isPalindrome(String s) {String str = s.toLowerCase();StringBuilder sb = new StringBuilder();StringBuilder rev = new StringBuilder();int i=0,j=s.length()-1;while(i<=j){while(i<j&&!(('a'<=str.charAt(i)&&str.charAt(i)<='z')||('0'<=str.charAt(i)&&str.charAt(i)<='9'))){i++;}while(i<j&&!(('a'<=str.charAt(j)&&str.charAt(j)<='z')||('0'<=str.charAt(j)&&str.charAt(j)<='9'))){j--;}sb.append(str.charAt(i));rev.append(str.charAt(j));i++;j--;}if(sb.toString().equals(rev.toString())){return true;}return false;}}

正常遍历，提取有用的字符串，反序然后做比较。

public class Solution {public boolean isPalindrome(String s) {String str = s.toLowerCase();StringBuilder sb = new StringBuilder();StringBuilder rev = new StringBuilder();for(int i=0;i<str.length();i++){char c = str.charAt(i);if(('a'<=c&&c<='z')||('0'<=c&&c<='9')){sb.append(c);}}for(int i=sb.length()-1;i>=0;i--){rev.append(sb.charAt(i));}if(sb.toString().equals(rev.toString())){return true;}return false;}}