var longestPalindrome = function(s) {    
    
    if(s.length<2){
        return s
    }
    
    let newString = '#'
    for(let i=0; i<s.length; i++){
        newString+=s[i]+'#'
    }
    
    let mid = Math.floor(newString.length/2);
    let rightStand = mid + 1;
    let leftStand = mid -1;
    let max = 0;
    let ans = '';
    let canMax = newString.length;
      
    const getMaxOrRenewMax = (stand, canMax) => {
        let left = stand;
        let right = stand;
        let count = 1;
        while(newString[--left] && newString[++right]){
            if(newString[left]===newString[right]){
                count+=2
            }else{
                break;
            }
        }
        
        if(max < count){
            max = count;
            ans = newString.substring(left+1, right)
        }
        if(count ===canMax){
            return ans.split('').filter(i=>i!=='#').join('')
        }
    }
    
    const midMax = getMaxOrRenewMax(mid, canMax);
    if(midMax) return midMax;
    canMax-=2;
           
    while(leftStand >= 0){
        const leftMax = getMaxOrRenewMax(leftStand, canMax);
        const rightMax = getMaxOrRenewMax(rightStand, canMax);
        if(leftMax) return leftMax;
        if(rightMax) return rightMax;
        canMax-=2;
        leftStand--;
        rightStand++;
    }
        
    return ans.split('').filter(i=>i!=='#').join('');

};