[Leetcode] 125. Valid Palindrome 解题报告

题目：

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.

思路：

这是一道名副其实的水题。然而如果了解了STL中的库函数之后，会让你写起来更得心应手。这里总结几个STL中的相关函数吧：

int isdigit(int c): check if a character is decimal digit. Decimal digits are any of: 0 1 2 3 4 5 6 7 8 9 

int isalpha(int c): check if a character is alphabetic. Checks whether c is an alphabetic letter, either uppercase or lowercase.

int isalnum(int c): check if a character is alphanumeric. Checks whether c is either a decimal digit or an uppercase or lowercase letter.

代码：

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