[leetcode]Valid Palindrome

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. 

算法思路：

思路1：首尾两指针，在原来的字符串的基础上，进行扫描判断。空间复杂度O(1)，时间O(n)，一次扫描。

public class Solution {
      public boolean isPalindrome(String s) {
        s = s.trim().toLowerCase();
        int i = 0;
        int j = s.length() - 1;
        while(i < s.length() && !isValid(s.charAt(i))) i++;
        while(j >= 0 && !isValid(s.charAt(j))) j--;
        while(i <= j){
            char a = s.charAt(i);
            char b = s.charAt(j);
            if(a != b){
                return false;
            }else{
                i++;
                while(i < s.length() && !isValid(s.charAt(i))) i++;
                j--;
                while(j >= 0 && !isValid(s.charAt(j))) j--;
            }
        }
        return true;
    }
    private boolean isValid(char c){
        if((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') ) return true;
        return false;
    }
}

思路2：第一遍扫描，将合法字符保留在另一个字符串s中，第二遍扫描s。空间复杂度O(1)，时间复杂度O(n)，两次扫描。

比较简单，不实现了