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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 注意 extern int toupper(int c); 如果c为小写英文字母，则返回对应的大写字母；否则返回原来的值。

class Solution {    public:        bool isPalindrome(string s) {            if(s.empty())                return true;            int idx1 = 0;            int idx2 = s.size()-1;            while(idx1 < idx2)            {                while(!isalnum(s[idx1]) && idx1 < idx2)                {                    idx1 ++;                }                while(!isalnum(s[idx2]) && idx1 < idx2)                {                    idx2 --;                }                if(idx1 >= idx2)                    break;                if(toupper(s[idx1]) == toupper(s[idx2]))                {                    idx1 ++;                    idx2 --;                }                else                    return false;            }            return true;        }};

 

使用transform 来更改字母的大小写

class Solution {    public:        bool isPalindrome(string s) {            transform(s.begin(), s.end(), s.begin(), ::tolower);            auto left = s.begin(), right = prev(s.end());            while (left < right) {                if (!::isalnum(*left)) ++left;                else if (!::isalnum(*right)) --right;                else if (*left != *right) return false;            }            return true;        }};