把根节点移动到槽位置

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/**
 * Ensures that the given root is the first node of its bin.
 */
static <K,V> void moveRootToFront(Node<K,V>[] tab, TreeNode<K,V> root) {
    int n;
    if (root != null && tab != null && (n = tab.length) > 0) {
        int index = (n - 1) & root.hash;
        TreeNode<K,V> first = (TreeNode<K,V>)tab[index];
      // root与fist不相同
      // 把root从链表中取出来，放到槽位置，root的next指向原来的first
      // ps: 这样改变了原链表的顺序
        if (root != first) {
            Node<K,V> rn; // root next
            tab[index] = root;
            TreeNode<K,V> rp = root.prev;
            if ((rn = root.next) != null)
                ((TreeNode<K,V>)rn).prev = rp;
            if (rp != null)
                rp.next = rn;
            if (first != null)
                first.prev = root;
            root.next = first;
            root.prev = null;
        }
        assert checkInvariants(root);
    }
}