LeetCode: 125. Valid Palindrome

题目描述

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.

题目大意: 判断指定字符串中的数字和字母序列是否是回文串。

解题思路

用左右两根指针(i, j)，分别从字符串左右出发，遇到字符或数字后，跟另外一根指针指向的值进行比较。如果不匹配，则不是回文串，当指针相遇后，还没有不匹配情况，则是回文串。

AC 代码

class Solution {
public:
    bool isPalindrome(string s) {
        int i = 0, j = s.size() -1;
        while(i < j)
        {
            while(i < j && !isalnum(s[i])) ++i;
            while(i < j && !isalnum(s[j])) --j;

            if(tolower(s[i++]) != tolower(s[j--])) return false;
        }

        return true;
    }
};