125. Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:

Input: "A man, a plan, a canal: Panama"
Output: true

Example 2:

Input: "race a car"
Output: false

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

C++兩個常用函數isalnum判斷char是否爲數字或者字母，是的話返回true。

toupper將小寫字母轉換爲大寫字母。