Valid Palindrome -- leetcode

Valid Palindrome -- leetcode

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.

基本思路：

題目要求，判斷一字符串，是否為符號，只考慮字母和數字，並且忽略大小寫。

用 isalnum 判斷是否為字母和數字；

用tolower將基本都作轉換小寫字母，從而在比較時忽略掉大小寫。

設置一頭一尾兩個指針，向中間移動。

此代碼在leetcode上實際執行時間為14ms。

class Solution {
public:
    bool isPalindrome(string s) {
        int left = 0;
        int right = s.size()-1;
        while (left < right) {
            if (!isalnum(s[left]))
                ++left;
            else if (!isalnum(s[right]))
                --right;
            else if (tolower(s[left++]) != tolower(s[right--]))
                return false;
        }
        return true;
    }
};