LeetCode (16) Valid Palindrome (回文)

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.

本题即英文版的“回文”，也就是一个字符串的字符顺序正向和反向遍历是一样的。本题要求的字符串只需要考虑字母和数字。题目的解法即通过使用两个指针/index，同时从前向后和从后向前遍历，并比较有效字母（字母和数字）是否相同。

class Solution {public:    bool isAlphaNum(char c)    {        if ( (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'))            return true;        else if (c >= '0' && c <= '9')            return true;        else            return false;    }    bool isPalindrome(string s) {        int i = 0, j = s.size() - 1;        while(i<j)        {            while(i < s.size() && !isAlphaNum(s[i]))                i++;            while(j >= 0 && !isAlphaNum(s[j]))                j--;            if (i < j && tolower(s[i]) != tolower(s[j]))                return false;            i++;            j--;        }        return true;    }};