[leetcoide] 【字符串】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.

题意

判断一个字符串是否是回文字符串，只考虑字母数字。

比如"A man, a plan, a canal: Panama"是回文；

"race a car" 不是回文。   空字符串也是回文。

题解

两个指针分别指向头和尾，注意只考虑字母数字，所以需要过滤。我这里自己写了过滤函数，c++里面也有专门的::isalnum(*left)

然后向中间夹逼即可。

class Solution {public:    bool isalpahanumeric(char c)    {        if((c>='0'&&c<='9')||(c>='a'&&c<='z'))            return true;        else return false;    }    bool isPalindrome(string s) {        transform(s.begin(),s.end(),s.begin(),::tolower);        auto left=s.begin(),right=prev(s.end());        while(left<right)        {            if(!isalpahanumeric(*left)) left++;            else if(!isalpahanumeric(*right)) right--;            else if(*left!=*right) return false;            else            {                left++;                right--;            }        }        return true;    }};