125. Valid Palindrome.md

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一天一道LeetCode

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##（一）题目

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.

##（二）解题

题目大意：判断一个字符串是不是有效的回文字符串。忽略里面除了数字和字母的其他字符。

解题思路：两个指针i和j，i从前往后，j从后往前，碰到p[i]和p[j]都是字母或者数字就比较大小，如果相等就i++，j--反之则返回false

直到i>=j时，说明是有效回文，返回true。

class Solution {

public:

    bool isPalindrome(string s) {

        int len = s.length();

        int i = 0;

        int j = len-1;

        while(i<=j)

        {

            while(!isalnum(s[i])&&i<=j) i++;//直到s[i]为字母或数字为止

            while(!isalnum(s[j])&&i<=j) j--;//直到s[j]为字母或数字为止

            if(i<=j&&tolower(s[i])!=tolower(s[j])) return false;//如果不等就返回false

            i++;j--;//反之就继续比较

        }

        return true;

    }

};