LeetCode: Valid Palindrome [125]

【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.

【题意】

给定一个字符串，仅仅关注英文字母和数字，忽略其它字符。问英文字母和数字按序排列后是否构成是回文

【思路】

利用回文串推断的老办法就可以，两个指针p1, p2分别从头和尾中间扫描，推断对称位置的字符是否同样，仅仅只是须要跳过除英文字母和数字之外的其它字符。

    另外还须要注意处理2中特殊情况：（1）字符串为空串，（2）字符串不是空串，但里面没有英文字母或者数字。 这两种情况都判定为true

【代码】

class Solution {
public:
    bool isAlphanumeric(char c){
        if(isdigit(c))return true;
        if(c>='A'&&c<='Z'||c>='a'&&c<='z')return true;
        return false;
    }
 
    bool isEqual(char c, char b){
        if(isdigit(c))return c==b;
        if(c>='A'&&c<='Z')c='a'+(c-'A');
        if(b>='A'&&b<='Z')b='a'+(b-'A');
        return c==b;
    }
 
    bool isPalindrome(string s) {
        int len=s.length();
        if(len==0)return true;
 
        int front=0;
        int back=len-1;
        while(front<back){
            while(front<=back && !isAlphanumeric(s[front]))front++;
            while(front<=back && !isAlphanumeric(s[back]))back--;
            if(front<=back){
                if(!isEqual(s[front], s[back]))return false;
                front++;
                back--;
            }
        }
        return true;
    }
};