LeetCode 125. Valid Palindrome（校验对称）

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原题网址：https://leetcode.com/problems/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.

方法一：边推进便比较。

public class Solution {    private boolean isAlphanumeric(char ch) {        if ((ch>='a' && ch<='z') || (ch>='A' && ch<='Z') || (ch>='0' && ch<='9')) return true;        return false;    }    public boolean isPalindrome(String s) {        if (null == s || "".equals(s)) return true;        int i=0, j=s.length()-1;        while (i<j) {            char ic = Character.toLowerCase(s.charAt(i));            if (!isAlphanumeric(ic)) {                i++;                continue;            }            char jc = Character.toLowerCase(s.charAt(j));            if (!isAlphanumeric(jc)) {                j--;                continue;            }            if (ic != jc) return false;            i++;            j--;        }        return true;    }}

方法二：先推进再比较。

public class Solution {    public boolean isPalindrome(String s) {        if (s==null) return true;        char[] sa = s.toCharArray();        int len = 0;        for(int i=0; i<sa.length; i++) {            if (sa[i] >= 'A' && sa[i] <= 'Z') sa[len++] = (char)('a' + sa[i] - 'A');            else if ((sa[i] >='a' && sa[i] <='z') || (sa[i] >= '0' && sa[i] <= '9')) sa[len++] = sa[i];        }        for(int i=0, j=len-1; i<j; i++, j--) {            if (sa[i] != sa[j]) return false;        }        return true;    }}

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