const _isPalindrome = string => {
                // 补全代码
                let left = 0;
                let right = string.length-1;
                let strL = null;
                let strR = null;
                while(left<=right){
                     strL = string[left];
                     strR = string[right];
                    if(strL!==strR){
                        return false;
                    }else{
                        left++;
                        right--;
                    }
                }
                return true;
            }