109. Convert Sorted List to Bina

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.
由于需要平衡树，所以取中点作为根，两边为左右子树，依次递归下去

var sortedListToBST = function(head) {
    if(head===null) return null;
    return toBST(head,null);
    function toBST(head, tail){
        var slow = head;
        var fast = head;
        if(head===tail) return null;
        while(fast!==tail&&fast.next!==tail){
            fast = fast.next.next;
            slow = slow.next;
        }
        var thead = new TreeNode(slow.val);
        thead.left = toBST(head,slow);
        thead.right = toBST(slow.next,tail);
        return thead;
    }
};