SOURCE

var longestPalindrome = function(s) {
    var len = s.length;
    if(len == 1){
        return s;
    }else if(len == 2){
        return s[0] === s[1] ? s : s[0];
    }
    var result = '';
    var startindex = 0 ,endindex = 2;
    for(var i = 2; i < len; i++){
        if(s[i] != s[i - 1] && s[i] != s.charAt(startindex)){
            if(i == len - 1){
                if(s[i - 1] == s[i - 2]){
                    if(i > 2){
                        s.substring(startindex + 1 ,i).length > result.length ? result = s.substring(startindex + 1 ,i) : '';
                    }else{
                        s.substring(0 ,2).length > result.length ? result = s.substring(0 ,2) : '';
                    }
                }else{
                    s[0].length > result.length ? result = s[0] : '';
                }
            }else{
                if(s[i - 1] == s[i - 2]){
                    s.substring(startindex ,i).length > result.length ? result = s.substring(startindex ,i) : '';
                    startindex = i - 1;
                }else{
                    startindex ++;
                }
                endindex ++;
            }
        }else if(s[i] == s[i - 1] && s[i] == s.charAt(startindex)){
            if(i == len - 1){
                s.length > result.length ? result = s : '';
            }else{
                endindex ++;
            }
        }else if(s[i] != s[i - 1] && s[i] == s.charAt(startindex)){
            s.substring(startindex ,endindex + 1).length > result.length ? result = s.substring(startindex ,endindex + 1) : '';
            startindex = i - 1;
            endindex ++;
        }else if(s[i] == s[i - 1] && s[i] != s.charAt(startindex)){
            if(i == len - 1){
                s.substring(startindex + 1 ,i + 1).length > result.length ? result = s.substring(startindex + 1 ,i + 1) : '';
            }else{
                endindex ++;
            }
        }
    }
    return result;
};
console.log(longestPalindrome('abcba'));

/*
    cbbd
    qfv
    ddd
    dd
    cdc
    mgmgqcccccccqbccccbxoooooooooooxo
    a
    ab
    dcccd
    yyyy1yyyyyy
*/