【Leetcode】【Easy】Valid Palindrome

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.

解题思路：

建立两个index，同时从前从后遍历字符串。遇到非字符需要跳过，遇到大写字母将其转为小写字母。比较两个index指向的字母，如果一致则继续遍历下一组，如果不相同则返回false，最后返回true；

注意：

1、非字母不需要比较，要跳过；

2、注意统一大小写；

 class Solution {
 public:
     bool isPalindrome(string s) {
         int len = s.length();
         int front = ;
         int behind = len - ;

         if (!len)
             return true;

         while (front <= behind) {

             if (!isAlphanumeric(&s[front])) {
                 front++;
                 continue;
             }

             if (!isAlphanumeric(&s[behind])) {
                 behind--;
                 continue;
             }

             if (s[front] != s[behind]) {
                 return false;
             } else {
                 front ++;
                 behind --;
             }

         }

         return true;
     }

     bool isAlphanumeric(char *s) {
         if ((*s>='a' && *s<='z') || (*s>='' && *s<=''))
             return true;
         if (*s>='A' && *s<='Z') {
             *s += ;
             return true;
         }
         return false;
     }

 };

另，有些程序直接使用了C++中isalnum()和tolower()函数，思路是一样的：

 class Solution {
 public:
     bool isPalindrome(string s) {
         int len = s.length();
         int front = ;
         int behind = len - ;

         if (!len)
             return true;

         while (front <= behind) {
             if (!isalnum(s[front])) {
                 front++;
                 continue;
             }

             if (!isalnum(s[behind])) {
                 behind--;
                 continue;
             }

             if (tolower(s[front]) != tolower(s[behind])) {
                 return false;
             } else {
                 front ++;
                 behind --;
             }
         }

         return true;
     }
 };

附录：

常用字符ASCII值