Data Structure —— Linkedlist（queue & stack in Java）

LinkedList其实也就是我们在数据结构中的链表，这种数据结构有这样的特性：

分配内存空间不是必须是连续的；
插入、删除操作很快，只要修改前后指针就OK了，时间复杂度为O(1)；
访问比较慢，必须得从第一个元素开始遍历，时间复杂度为O(n)；

在Java中，LinkedList提供了丰富的方法，可以模拟链式队列，链式堆栈等数据结构，为用户带来了极大的方便，下面看看这些方法的用法：

add

boolean add(E e)：在链表后添加一个元素，如果成功，返回true，否则返回false；
void addFirst(E e)：在链表头部插入一个元素；
addLast(E e)：在链表尾部添加一个元素；
void add(int index, E element)：在指定位置插入一个元素。

下面是代码演示：

public class LinkedListMethodsDemo {
    public static void main(String[] args) {
        LinkedList<String> linkedList = new LinkedList<>();

        linkedList.add("first");
        linkedList.add("second");
        linkedList.add("third");
        System.out.println(linkedList);

        linkedList.addFirst("addFirst");
        System.out.println(linkedList);

        linkedList.addLast("addLast");
        System.out.println(linkedList);

        linkedList.add(2, "addByIndex");
        System.out.println(linkedList);
    }
}

输出：

[first, second, third]
[addFirst, first, second, third]
[addFirst, first, second, third, addLast]
[addFirst, first, addByIndex, second, third, addLast]

remove

E remove()；移除链表中第一个元素；
boolean remove(Object o)：移除链表中指定的元素；
E remove(int index)：移除链表中指定位置的元素；
E removeFirst()：移除链表中第一个元素，与remove类似；
E removeLast()：移除链表中最后一个元素；
boolean removeFirstOccurrence(Object o)：移除链表中第一次出现所在位置的元素；
boolean removeLastOccurrence(Object o)：移除链表中最后一次出现所在位置的元素；

public class LinkedListMethodsDemo {
    public static void main(String[] args) {
        LinkedList<String> linkedList = new LinkedList<>();

        linkedList.add("first");
        linkedList.add("second");
        linkedList.add("second");
        linkedList.add("third");
        linkedList.add("four");
        linkedList.add("five");
        System.out.println(linkedList);

        linkedList.remove();
        System.out.println("remove: " + linkedList);

        linkedList.remove("second");
        System.out.println("remove(Object): " + linkedList);

        linkedList.remove("six");
        System.out.println("remove(Object) not exist: " + linkedList);

        linkedList.remove(2);
        System.out.println("remove(index): " + linkedList);

        linkedList.removeFirst();
        System.out.println("removeFirst: " + linkedList);

        linkedList.removeLast();
        System.out.println("removeLast:" + linkedList);

        System.out.println("----------------------------------");
        linkedList.clear();

        linkedList.add("first");
        linkedList.add("second");
        linkedList.add("first");
        linkedList.add("third");
        linkedList.add("first");
        linkedList.add("five");
        System.out.println(linkedList);

        linkedList.removeFirstOccurrence("first");
        System.out.println("removeFirstOccurrence: " + linkedList);

        linkedList.removeLastOccurrence("first");
        System.out.println("removeLastOccurrence: " + linkedList);
    }
}

输出：

[first, second, second, third, four, five]
remove: [second, second, third, four, five]
remove(Object): [second, third, four, five]
remove(Object) not exist: [second, third, four, five]
remove(index): [second, third, five]
removeFirst: [third, five]
removeLast:[third]
----------------------------------
[first, second, first, third, first, five]
removeFirstOccurrence: [second, first, third, first, five]
removeLastOccurrence: [second, first, third, five]

get

E get(int index)：按照下边获取元素；
E getFirst()：获取第一个元素；
E getLast()：获取第二个元素；

public class LinkedListMethodsDemo {
    public static void main(String[] args) {
        LinkedList<String> linkedList = new LinkedList<>();

        linkedList.add("first");
        linkedList.add("second");
        linkedList.add("second");
        linkedList.add("third");
        linkedList.add("four");
        linkedList.add("five");
        System.out.println(linkedList);

        linkedList.get(3);
        System.out.println("get(index): " + linkedList.get(3));

        linkedList.getFirst();
        System.out.println("getFirst: " + linkedList.getFirst());

        linkedList.getLast();
        System.out.println("getLast: " + linkedList.getLast());

        System.out.println(linkedList);
    }
}

输出：

[first, second, second, third, four, five]
get(index): third
getFirst: first
getLast: five
[first, second, second, third, four, five]

注意到了，链表前后没有产生变化。

push、pop、poll

void push(E e)：与addFirst一样，实际上它就是addFirst；
E pop()：与removeFirst一样，实际上它就是removeFirst；
E poll()：查询并移除第一个元素；

public class LinkedListMethodsDemo {
    public static void main(String[] args) {
        LinkedList<String> linkedList = new LinkedList<>();

        linkedList.push("first");
        linkedList.push("second");
        linkedList.push("second");
        linkedList.push("third");
        linkedList.push("four");
        linkedList.push("five");
        System.out.println("linkedList: " + linkedList);

        System.out.println("pop: " + linkedList.pop());
        System.out.println("after pop: " + linkedList);

        System.out.println("poll: " + linkedList.poll());
        System.out.println("after poll: " + linkedList);
    }
}

输出：

linkedList: [five, four, third, second, second, first]
pop: five
after pop: [four, third, second, second, first]
poll: four
after poll: [third, second, second, first]

通过代码示例可以看出：push，pop的操作已经很接近stack的操作了。

如果链表为空的时候，看看poll与pop是啥区别：

public class LinkedListMethodsDemo {
    public static void main(String[] args) {
        LinkedList<String> linkedList = new LinkedList<>();

        System.out.println("poll: " + linkedList.poll());
        System.out.println("pop: " + linkedList.pop());
    }
}

输出：

poll: null
Exception in thread "main" java.util.NoSuchElementException
    at java.util.LinkedList.removeFirst(LinkedList.java:270)
    at java.util.LinkedList.pop(LinkedList.java:801)
    at org.ks.algorithm.LinkedListMethodsDemo.main(LinkedListMethodsDemo.java:13)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

可以看出poll返回null，而pop则产生异常。

peek

E peek()：获取第一个元素，但是不移除；
E peekFirst()：获取第一个元素，但是不移除；
E peekLast()：获取最后一个元素，但是不移除；

public class LinkedListMethodsDemo {
    public static void main(String[] args) {
        LinkedList<String> linkedList = new LinkedList<>();

        linkedList.push("first");
        linkedList.push("second");
        linkedList.push("second");
        linkedList.push("third");
        linkedList.push("four");
        linkedList.push("five");
        System.out.println("linkedList: " + linkedList);

        System.out.println("peek: " + linkedList.peek());
        System.out.println("peekFirst: " + linkedList.peekFirst());
        System.out.println("peekLast: " + linkedList.peekLast());

        System.out.println("linkedList: " + linkedList);
    }
}

输出：

linkedList: [five, four, third, second, second, first]
peek: five
peekFirst: five
peekLast: first
linkedList: [five, four, third, second, second, first]

如果没找到对应的元素，统统输出null：

public class LinkedListMethodsDemo {
    public static void main(String[] args) {
        LinkedList<String> linkedList = new LinkedList<>();

        System.out.println("linkedList: " + linkedList);
        System.out.println("peek: " + linkedList.peek());
        System.out.println("peekFirst: " + linkedList.peekFirst());
        System.out.println("peekLast: " + linkedList.peekLast());
    }
}

输出：

linkedList: []
peek: null
peekFirst: null
peekLast: null

offer

boolean offer(E e)：在链表尾部插入一个元素；
boolean offerFirst(E e)：与addFirst一样，实际上它就是addFirst；
boolean offerLast(E e)：与addLast一样，实际上它就是addLast；

public class LinkedListMethodsDemo {
    public static void main(String[] args) {
        LinkedList<String> linkedList = new LinkedList<>();

        linkedList.push("first");
        linkedList.push("second");
        linkedList.push("second");
        linkedList.push("third");
        linkedList.push("four");
        linkedList.push("five");
        System.out.println("linkedList: " + linkedList);

        linkedList.offer("six");
        System.out.println("linkedList: " + linkedList);

        linkedList.offerFirst("zero");
        System.out.println("linkedList: " + linkedList);

        linkedList.offerLast("seven");
        System.out.println("linkedList: " + linkedList);
    }
}

输出：

linkedList: [five, four, third, second, second, first]
linkedList: [five, four, third, second, second, first, six]
linkedList: [zero, five, four, third, second, second, first, six]
linkedList: [zero, five, four, third, second, second, first, six, seven]

其他

LinkedList中常用的方法基本都列出来了，当然还有一些其他的例子，这里就一起演示了：

public class LinkedListMethodsDemo {
    public static void main(String[] args) {
        LinkedList<String> linkedList = new LinkedList<>();

        linkedList.push("first");
        linkedList.push("second");
        linkedList.push("second");
        linkedList.push("third");
        linkedList.push("four");
        linkedList.push("five");
        System.out.println("linkedList: " + linkedList);

        System.out.println("linkedList.contains(\"second\"): " + linkedList.contains("second"));
        System.out.println("linkedList.contains(\"six\"): " + linkedList.contains("six"));
        System.out.println("linkedList.element(): " + linkedList.element());
        System.out.println("linkedList: " + linkedList);
        System.out.println("linkedList.set(3, \"set\"): " + linkedList.set(3, "set"));
        System.out.println("linkedList: " + linkedList);
        System.out.println("linkedList.subList(2,4): " + linkedList.subList(2,4));
        System.out.println("linkedList: " + linkedList);
    }
}

输出：

linkedList: [five, four, third, second, second, first]
linkedList.contains("second"): true
linkedList.contains("six"): false
linkedList.element(): five
linkedList: [five, four, third, second, second, first]
linkedList.set(3, "set"): second
linkedList: [five, four, third, set, second, first]
linkedList.subList(2,4): [third, set]
linkedList: [five, four, third, set, second, first]

从输出可以看出，除了set改变原linkedlist，其他几个方法都不改变原链表。

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Data Structure —— Linkedlist（queue & stack in Java）

add

remove

get

push、pop、poll

peek

offer

其他

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