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125. Valid Palindrome

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.

 

去掉 non-alphanumeric 的字符,然后忽略大小写判断是不是回文串。

two pointers

class Solution {public:    bool isPalindrome(string s) {        string::const_iterator left = s.cbegin();        string::const_iterator right = s.cend() - 1;        while (left < right) {            while (left < right && !isalnum(*left)) left++;            while (right > left && !isalnum(*right)) right--;            if (tolower(*left) != tolower(*right)) return false;            else {                left++;                right--;            }        }        return true;    }};

 

125. Valid Palindrome