LeetCode——Valid Palindrome

Description：

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.

public class Solution {
public boolean isPalindrome(String s) {
StringBuilder str = new StringBuilder();
for(int i=0; i<s.length(); i++) {
if(s.charAt(i)>='a'&&s.charAt(i)<='z' || s.charAt(i)>='A'&&s.charAt(i)<='Z'
|| s.charAt(i)>='0'&&s.charAt(i)<='9') {
str.append(s.charAt(i));
}
}
str = new StringBuilder(str.toString().toLowerCase());
int flag = 1;
for(int i=0; i<str.length()/2; i++) {
if(str.charAt(i) != str.charAt(str.length()-i-1)) {
flag = 0;
break;
}

}
if(flag == 1)
return true;
else
return false;

}
}