問題描寫

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.

回文字符串判斷，但只判斷字母和數字是不是構成回文字符串，標點符號被略過。

問題分析

保存左右兩個指針，每輪先移動左指針獲得1個字母或數字，其它字符都跳過；然后移動右指針，找到1個字母或數字，其它字符都跳過；如果此時左右兩個字符都為字母或數字并但是兩個不相等，則返回false；進行下輪指針移動直到兩個指針相等。

代碼

class Solution {
public:
    bool isPalindrome(string s) {
        if(s.length()==0||s.length()==1) return true;
        int left=0, right=s.length()-1;
        // 大寫轉小寫
        for(int i=0;i<s.length();i++) {
            char ch = s[i];
            if(ch>='A'&&ch<='Z') ch = 'a' + ch - 'A';
            s[i] = ch;
        }
        while(left<right) {
            char l = s[left++];
            char r = s[right--];
            while(!((l>='a'&&l<='z')||(l>='0'&&l<='9'))&&left<s.length()) {
                l = s[left++];
            }
            while(!((r>='a'&&r<='z')||(r>='0'&&r<='9'))&&right>0) {
                r = s[right--];
            }
            if(((l>='a'&&l<='z')||(l>='0'&&l<='9'))&&((r>='a'&&r<='z')||(r>='0'&&r<='9'))&&r!=l) {
                return false;
            }
        }
        return true;
    }
};