public:
 bool judge(string str) {
 int left = 0; //⾸
 int right = str.length() - 1; //尾
 while(left < right){ //⾸尾往中间靠
 if(str[left] != str[right]) //⽐较前后是否相同
 return false;
 left++;
 right--;
 }
 return true;
 }
};