LeetCode – Valid Palindrome (Java)

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.

Thoughts

From start and end loop though the string, i.e., char array. If it is not alpha or number, increase or decrease pointers. Compare the alpha and numeric characters. The solution below is pretty straightforward.

Java Solution 1

public class Solution {     public  boolean isPalindrome(String s) {         if(s == null) return false;        if(s.length() < 2) return true;         char[] charArray = s.toCharArray();        int len = s.length();         int i=0;        int j=len-1;         while(i<j){            char left =  charArray[i];            char right = charArray[j];             while(i<len-1 && !isAlpha(left) && !isNum(left)){                i++;                left =  charArray[i];            }             while(j>0 && !isAlpha(right) && !isNum(right)){                j--;                right = charArray[j];            }             if(i >= j)            break;             left =  charArray[i];            right = charArray[j];             if(!isSame(left, right)){                return false;            }             i++;            j--;        }        return true;    }     public  boolean isAlpha(char a){        if((a >= 'a' && a <= 'z') || (a >= 'A' && a <= 'Z')){            return true;        }else{            return false;        }    }     public  boolean isNum(char a){        if(a >= '0' && a <= '9'){            return true;        }else{            return false;        }    }     public  boolean isSame(char a, char b){        if(isNum(a) && isNum(b)){            return a == b;        }else if(Character.toLowerCase(a) == Character.toLowerCase(b)){            return true;        }else{            return false;        }    }}

Java Solution 2 – Remove Non-Letter First

This solution removes the special characters first. (Thanks to Tia)

public boolean isPalindrome(String s) {s = s.replaceAll("[^a-zA-Z0-9]", "").toLowerCase(); int len = s.length();if (len < 2)return true; Stack<Character> stack = new Stack<Character>(); int index = 0;while (index < len / 2) {stack.push(s.charAt(index));index++;} if (len % 2 == 1)index++; while (index < len) {if (stack.empty())return false; char temp = stack.pop();if (s.charAt(index) != temp)return false;elseindex++;} return true;}