125. 验证回文串

方法一

class Solution {
public:
    bool isPalindrome(string s) {
        string newStr = "";
        for(int fast = 0; fast < s.size(); fast++){
            if(isalnum(s[fast]))){
                newStr += tolower(s[fast]);
            }
        }
        string tmp = newStr;
        reverse(tmp.begin(), tmp.end());
        if(strcmp(newStr.c_str(), tmp.c_str()) == 0) {
            return true;
        }
        return false;
    }
};

class Solution {
public:
    bool isPalindrome(string s) {
        string newStr = "";
        for(char ch : s){
            if(isalnum(ch)){
                newStr += tolower(ch);
            }
        }
        string newStr_rev(newStr.rbegin(),newStr.rend());
        return newStr == newStr_rev;
    }
};

方法二

class Solution {
public:
    bool isPalindrome(string s) {
        string newStr = "";
        for(int fast = 0; fast < s.size(); fast++){
            if(isalnum(s[fast]))){
                newStr += tolower(s[fast]);
            }
        }
        for(int front = 0,rear = newStr.size()-1; front  < newStr.size()/2; front++,rear--){
            if(newStr[front] != newStr[rear]){
                return false;
            }
        }
        return true;
    }
};

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

方法三

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