125. Valid Palindrome

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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.

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Seen this question in a real interview before?   

Yes

 

分析：有效的回文

给定一个字符串，判断是否是回文，只考虑字母数字，并且忽略大小写。

这道题唯一难点就是处理大小写和非数字字母的字符。其实c++有现成的函数

isalnum() 判断字符是否为数字或者字母

toupper()将字母变成大写

tolower()将字母变成小写

代码：

class Solution {public:    bool isPalindrome(string s) {        int first,last;first = 0;last = s.length()-1;while(first<=last){while(!(isalnum(s[first])) && first < last)first++;while(!(isalnum(s[last])) && first < last)last--;if(toupper(s[first])!=toupper(s[last]))return false;last--;first++;}return true;    }};