编辑代码

function longestPalindrome(str){
    let start=0;
    let maxlength=1;
    if(str.length<2){
        return str
    }
    for(let i=0;i<str.length;i++){
        expandAroundCenter(i,i+1)
        expandAroundCenter(i-1,i+1)
    }
    function expandAroundCenter(left,right){
        while(left>=0&&right<str.length&&str[left] === str[right]){
            if(right-left+1>maxlength){
                maxlength=right-left+1
                start=left
            }
            left--;
            right++;
        }
}
    return str.slice(start,start+maxlength)
}
console.log(longestPalindrome("abcdadcab"))