5. 最长回文子串（中等）

输入：s = "babad"
输出："bab"
解释："aba" 同样是符合题意的答案。

 示例2：

输入：s = "cbbd"
输出："bb"

 示例3：

输入：s = "a"
输出："a"

var longestPalindrome = function(s) {
    var expandAC = function(s,left,right){
        var list = []
        while(left>=0 && right<s.length && s.slice(left,left+1) == s.slice(right,right+1)){
            left = left-1;
            right = right+1;
        }
        list = [left+1,right-1]
        return list    //JS中不能写成return left+1,right-1;
    }
    var start=0,end=0;
    var left1,right1,left2,right2
    var list1,list2
    for(var i=0;ilist1 = expandAC(s,i,i);
        left1 = list1[0];
        right1 = list1[1];  //JS中不能写成 left1,right1 = list1[0],list2[1];
        list2 = expandAC(s,i,i+1);
        left2 = list2[0];
        right2 = list2[1];
        if(right1-left1>end-start){
            start = left1;
            end = right1;
        }
        if(right2-left2>end-start){
            start = left2;
            end = right2;
        }
    }
    return s.slice(start,end+1)
};