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.

去掉全部符号然后前后比较即可。 注意character包含字母和数字。

public class Solution {
    public boolean isPalindrome(String s) {
        // Start typing your Java solution below
        // DO NOT write main() function
        s = s.toLowerCase();
        StringBuffer ss = new StringBuffer();
        for(int i = 0; i < s.length(); i ++){
            if((s.charAt(i) - 'a' > -1 && s.charAt(i) - 'z' < 1) || (s.charAt(i) - '0' > -1 && s.charAt(i) - '9' < 1)){
                ss.append(s.charAt(i));
            }
        }
        for(int i = 0; i < ss.length() / 2; i ++){
            if(ss.charAt(i) != ss.charAt(ss.length() - 1 - i))return false;
        }
        return true;
    }
}

 不使用额外的空间：

public class Solution {
    public boolean isPalindrome(String s) {
        if(s == null) return true;
        s = s.toLowerCase();
        int i = 0, j = s.length() - 1;
        while(i < j){
            while(i < j && !(s.charAt(i) >= 'a' && s.charAt(i) <= 'z') && !(s.charAt(i) >= '0' && s.charAt(i) <= '9')) i ++;
            while(i < j && !(s.charAt(j) >= 'a' && s.charAt(j) <= 'z') && !(s.charAt(j) >= '0' && s.charAt(j) <= '9')) j --;
            if(s.charAt(i) != s.charAt(j))return false;
            else{i ++; j --;}
        }
        return true;
    }
}