[A] list: LinkedList

Author: c-rainstorm

java/source-code/jdk/collection/list-linkedlist.md

@@ -0,0 +1,64 @@
+# LinkedList
+
+## 时间复杂度
+
+## 下标操作
+
+任何带下标的操作操作基本都是 O(n/2) ~ O(n)
+
+```java
+/**
+    * Returns the (non-null) Node at the specified element index.
+    */
+Node<E> node(int index) {
+    // assert isElementIndex(index);
+
+    if (index < (size >> 1)) {
+        // 如果 index 小于 size 的一半，则从往后遍历
+        Node<E> x = first;
+        for (int i = 0; i < index; i++)
+            x = x.next;
+        return x;
+    } else {
+        // 如果 index 大于 size 的一半，则从后往前遍历
+        Node<E> x = last;
+        for (int i = size - 1; i > index; i--)
+            x = x.prev;
+        return x;
+    }
+}
+```
+
+## 双向链表
+
+```java
+/**
+* Pointer to first node.
+* Invariant: (first == null && last == null) ||
+*            (first.prev == null && first.item != null)
+*/
+transient Node<E> first;
+
+/**
+* Pointer to last node.
+* Invariant: (first == null && last == null) ||
+*            (last.next == null && last.item != null)
+*/
+transient Node<E> last;
+
+private static class Node<E> {
+    E item;
+    Node<E> next;
+    Node<E> prev;
+
+    Node(Node<E> prev, E element, Node<E> next) {
+        this.item = element;
+        this.next = next;
+        this.prev = prev;
+    }
+}
+```
+
+## 最后
+
+没啥好说的很简单