[leetcode] 125. Valid Palindrome

Question:

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.

Solution:

从左往右遍历直到遇到下一个字母或数字，再从右往左遍历直到遇到下一个字母或数字，顺便处理为小写字母。再进行比较，如果两字符不同则不是回文。如果遍历时，left >= right 则跳出并且认为是回文。
时间复杂度为O(n)，空间复杂度为O(1)。

class Solution {public:    bool isPalindrome(string s) {        if (s.empty()) {            return true;        }        int left = 0, right = s.size()-1;        while (left < right) {            while (!isAlphaNum(s[left])) {                left++;            }            while (!isAlphaNum(s[right])) {                right--;            }            if (left >= right) {                return true;            }            if (s[left] != s[right]) {                return false;            }            left++;            right--;        }        return true;    }    bool isAlphaNum(char & ch) {        if (ch >= 'A' && ch <= 'Z') {            ch = 'a' + ch - 'A';        }        return (ch >= 'a' && ch <= 'z') || (ch >= '0' && ch <= '9');    }};