1818```
1919
2020``` java
21- class TreeNode {
21+ import java.util.LinkedList ;
22+
23+ class TreeNode {
2224 public TreeNode left;
2325 public TreeNode right;
2426 public int value;
25- public TreeNode (int value ){
27+
28+ public TreeNode (int value ) {
2629 this . value = value;
2730 }
28-
31+
2932 public TreeNode invertNode (TreeNode root ) {
30- if (root == null ){
33+ if (root == null ) {
3134 return null ;
3235 }
33- TreeNode temp = root. left;
34- root. left = invertNode(root. right);
35- root. right = invertNode(temp);
36- return root;
36+ TreeNode temp = root. left;
37+ root. left = invertNode(root. right);
38+ root. right = invertNode(temp);
39+ return root;
40+ }
41+
42+ public void printTreeNode () {
43+ LinkedList<TreeNode > queue = new LinkedList<> ();
44+ queue. add(this );
45+
46+ TreeNode currentLineRightestNode = this ;
47+ TreeNode nextLineRightestNode = null ;
48+
49+ while (! queue. isEmpty()) {
50+ TreeNode currentNode = queue. poll();
51+
52+ if (currentNode. left != null ) {
53+ queue. add(currentNode. left);
54+ nextLineRightestNode = currentNode. left;
55+ }
56+ if (currentNode. right != null ) {
57+ queue. add(currentNode. right);
58+ nextLineRightestNode = currentNode. right;
59+ }
60+
61+ System . out. print(currentNode. value);
62+
63+ if (currentNode. value == currentLineRightestNode. value) {
64+ System . out. println();
65+ currentLineRightestNode. value = nextLineRightestNode. value;
66+ }
67+ }
68+ }
69+ }
70+
71+ public class ReverseBinaryTree {
72+ public static void main (String [] args ) {
73+ TreeNode root = new TreeNode (4 );
74+ root. left = new TreeNode (2 );
75+ root. right = new TreeNode (7 );
76+ root. left. left = new TreeNode (1 );
77+ root. left. right = new TreeNode (3 );
78+ root. right. left = new TreeNode (6 );
79+ root. right. right = new TreeNode (9 );
80+ root. printTreeNode();
81+ System . out. println(" -------"
82+ root. invertNode(root);
83+ root. printTreeNode();
3784 }
3885}
3986```
@@ -43,79 +90,56 @@ LRU，全称Least Recently Used，最近最少使用缓存。
4390
4491在设计数据结构时，需要能够保持顺序，且是最近使用过的时间顺序被记录，这样每个item的相对位置代表了最近使用的顺序。满足这样考虑的结构可以是链表或者数组，不过链表更有利于Insert和Delete的操纵。
4592
46- 此外，需要记录链表的head和tail，从而方便进行移动到tail或者删除head的操作：
47- head.next作为最近最少使用的item，tail.prev为最近使用过的item，
48-
49- 在set时，如果超出capacity，则删除head.next，同时将要插入的item放入tail.prev,
50- 在get时，如果存在，只需把item更新到tail.prev即可。
51- 这样set与get均为O(1)时间的操作 （HashMap Get/Set + LinkedList Insert/Delete)，空间复杂度为O(n), n为capacity。
52-
5393``` java
54- public class LRUCache {
55- private class Node {
56- Node prev;
57- Node next;
58- int key;
59- int value;
60-
61- public Node (int key , int value ) {
62- this . key = key;
63- this . value = value;
64- this . prev = null ;
65- this . next = null ;
66- }
67- }
94+ import java.util.HashMap ;
95+ import java.util.LinkedList ;
6896
97+ public class LRUCache2 {
98+ private HashMap<Integer , Integer > cacheMap = new HashMap<> ();
99+ private LinkedList<Integer > recentlyList = new LinkedList<> ();
69100 private int capacity;
70- private HashMap<Integer , Node > hm = new HashMap<Integer , Node > ();
71- private Node head = new Node (- 1 , - 1 );
72- private Node tail = new Node (- 1 , - 1 );
73101
74- // @param capacity, an integer
75- public LRUCache (int capacity ) {
102+ public LRUCache2 (int capacity ) {
76103 this . capacity = capacity;
77- this . head. next = this . tail;
78- this . tail. prev = this . head;
79104 }
80105
81- // @return an integer
82- public int get (int key ) {
83- if (! hm. containsKey(key)) {
106+ private int get (int key ) {
107+ if (! cacheMap. containsKey(key)) {
84108 return - 1 ;
85109 }
86- Node current = hm. get(key);
87- current. prev. next = current. next;
88- current. next. prev = current. prev;
89110
90- moveToTail(current);
111+ recentlyList. remove((Integer ) key);
112+ recentlyList. add(key);
91113
92- return hm . get(key). value ;
114+ return cacheMap . get(key);
93115 }
94116
95- // @param key, an integer
96- // @param value, an integer
97- // @return nothing
98- public void set (int key , int value ) {
99- if (get(key) != - 1 ) {
100- hm. get(key). value = value;
101- return ;
117+ private void put (int key , int value ) {
118+ if (cacheMap. containsKey(key)) {
119+ recentlyList. remove((Integer ) key);
120+ recentlyList. add(key);
102121 }
103- if (hm. size() == capacity) {
104- hm. remove(head. next. key);
105- head. next = head. next. next;
106- head. next. prev = head;
122+
123+ if (cacheMap. size() == capacity) {
124+ cacheMap. remove(recentlyList. removeFirst());
107125 }
108126
109- Node insert = new Node (key, value);
110- hm . put (key, insert );
111- moveToTail(insert);
127+ cacheMap . put (key, value);
128+ recentlyList . add (key);
129+
112130 }
113131
114- private void moveToTail (Node current ) {
115- current. next = tail;
116- tail. prev. next = current;
117- current. prev = tail. prev;
118- tail. prev = current;
132+ public static void main (String [] args ) {
133+ LRUCache2 cache = new LRUCache2 (2 );
134+ cache. put(1 , 1 );
135+ cache. put(2 , 2 );
136+ System . out. println(cache. get(1 )); // returns 1
137+ cache. put(3 , 3 ); // 驱逐 key 2
138+ System . out. println(cache. get(2 )); // returns -1 (not found)
139+ cache. put(4 , 4 ); // 驱逐 key 1
140+ System . out. println(cache. get(1 )); // returns -1 (not found)
141+ System . out. println(cache. get(3 )); // returns 3
142+ System . out. println(cache. get(4 )); // returns 4
119143 }
120144}
121145```
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