### LeetCode Challenge #125. Valid Palindrome

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

Given a string `s`, return `true` if it is a palindrome, or `false` otherwise.

Example 1:

```Input: s = "A man, a plan, a canal: Panama"
Output: true
Explanation: "amanaplanacanalpanama" is a palindrome.
```

Example 2:

```Input: s = "race a car"
Output: false
Explanation: "raceacar" is not a palindrome.
```

Example 3:

```Input: s = " "
Output: true
Explanation: s is an empty string "" after removing non-alphanumeric characters.
Since an empty string reads the same forward and backward, it is a palindrome.
```

Constraints:

• `1 <= s.length <= 2 * 105`
• `s` consists only of printable ASCII characters.
##### Video Solution
###### Java Solution
```				```
class Solution {
public boolean isPalindrome(String s) {
if(s.length()==1){
return true ;
}

StringBuilder sb = new StringBuilder("");

for(int i=0;i<s.length();i++){
char ch = s.charAt(i);

if      (ch>='a' && ch<='z'){sb.append(ch);}
else if ( ch>='A' && ch<='Z')
{
ch = (char)(ch-'A'+'a');
sb.append(ch);
}
else if ( ch>='0' && ch<='9')
{
sb.append(ch);
}
}

int start = 0 ;
int end = sb.length()-1;

while(start<end){
int sChar = sb.charAt(start);
int eChar = sb.charAt(end);
if(sChar!=eChar){
return false;
}
start++;
end--;
}

return true ;
}
}
```
```