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因为本身就有头尾指针，遍历元素的时候会从最近的指针出发，最多会遍历n/2个元素，平均应该是n/4。Java8源码片段如下：
Node<E> node(int index) {
// assert isElementIndex(index);
if (index < (size >> 1)) {
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}