125. Valid Palindrome

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.

分析

题目给出一个字符串，判断该字符串是不是回文，但是要忽略除了数字字母以外的任何字符，并且不区分大小写，而且有空字符串的情况出现，空字符串判定为真

解题的想法就是，先得到干净的字符串（只有字母数字），然后首位开始依次向中间移动开始判断

代码

class Solution {
public:
    bool isPalindrome(string s) {
        //判断字符串是不是为空，为空直接true 
        if(s.empty()) 
            return true;
        //去除除了字母和数字以外的所有字符 
        string str = clear(s);

        for (int i=0,j=str.length()-1;i<j;i++,j--){
            if (str[i]!=str[j]){
                return false;
            }
        }
        
        return true; 
    }
    
    //清理函数，将字符串去除除了字母和数字意外的字符，并将全部字符转为小写 
    string clear(string s){
        string str = "";
        char c;
        for(int i=0;i<s.length();i++){
            c = s[i];
            if (c>='A'&&c<='Z' || c>='a'&&c<='z' || c>='0'&&c<='9'){
                if (c>='A'&c<='Z')
                    str+=c+32;
                else
                    str+=c;
            }
        } 
        return str;
    } 
};