125. Valid Palindrome (java,)

125. Valid Palindrome

Description Submissions Solutions

• Total Accepted: 150777
• Total Submissions: 585174
• Difficulty: Easy
• Contributor: LeetCode

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.

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题目链接：https://leetcode.com/problems/valid-palindrome/#/description

解题思路：本来是想处理一遍字符串，过滤掉不合法的字符，但这样会超时，时间是o(n)，只能边处理边判断，一边从前往后过滤掉不合法字符，一边从后往前过滤掉不合法字符，然后前后对应字符比较是否相等。注意判断空字符串，时间是o(n/2)。

代码如下：

public class Solution {
    public boolean isPalindrome(String s) {
        s = s.toLowerCase();
		if(s.length() <= 1){
			return true;
		}
		int i=0,j= s.length()-1;
		while(i<j){
			char ch1='A' , ch2 ='A' ;
			while(i < j){
				ch1 = s.charAt(i);
				if(ch1>='0' && ch1 <='9' ||ch1 >= 'a' && ch1<='z'){
					break;
				}
				ch1='A';
				i++;
			}
			while(j>i ){
				ch2 = s.charAt(j);
				if(ch2 >='0' && ch2 <='9' ||ch2 >= 'a' && ch2<='z'){
					break;
				}
				j--;
				ch2 ='A';
			}
			if(ch1!='A' && ch2!='A' && ch1 != ch2)
			{
				return false;
			}
			i++;
			j--;
		}
		return true;
    }
}