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Leetcode：LRUCache四个版本实现

题目

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
set(key, value) - Set or insert the value if the key is not already present. When the cache reached its capacity, it should invalidate the least recently used item before inserting a new item.

链接：https://oj.leetcode.com/problems/lru-cache/

分析

1：维护最近最少（LRU）使用的cache

　　1）使用count计数，每次操作cache时（get、set），该cache的count置0，其余cache的count加1，count最大的为最近最少使用的cache

2）使用链表，每次操作cache时（get、set），将该cache移动至链表头，链表尾的cache为最近最少使用的cache

2：快速查找cache

　　1）使用stl::unordered_map存储cache地址（内部hashTable）

版本一

使用std::list维护LRU，链表中存储cache实际空间。

Runtime: 248ms。

 1 #include <list>
 2 #include <unordered_map>
 3 
 4 struct KeyValue {
 5     KeyValue(int pKey, int pValue):key(pKey), value(pValue) {};
 6     int key;
 7     int value;
 8 };
 9 
10 class LRUCache{
11 public:
12     LRUCache(int capacity):_capacity(capacity) {};
13     
14     int get(int key);
15     void set(int key, int value);
16     
17 private:
18     std::unordered_map<int, std::list<KeyValue>::iterator> keyToNodeItr;
19     std::list<KeyValue> lru;
20     
21     int _capacity;
22 };
23 
24 void LRUCache::set(int key, int value) {
25     auto itr = keyToNodeItr.find(key);
26     if (itr != keyToNodeItr.end()) { // set value
27         itr->second->value = value;
28         KeyValue tmp(itr->second->key, itr->second->value);
29         keyToNodeItr.erase(itr->second->key);
30         lru.erase(itr->second);
31         lru.push_front(tmp);
32     } else { // insert value
33         if (lru.size() != _capacity) {
34             lru.push_front(KeyValue(key, value));
35         } else {
36             // pop back lru
37             if (lru.size() != 0) {
38                 keyToNodeItr.erase((lru.rbegin())->key);
39                 lru.pop_back();
40             }
41             lru.push_front(KeyValue(key, value));
42         }
43     }
44     
45     keyToNodeItr.insert(std::pair<int, std::list<KeyValue>::iterator>(key, lru.begin()));
46 }
47 
48 int LRUCache::get(int key) {
49     auto itr = keyToNodeItr.find(key);
50     if (itr != keyToNodeItr.end()) {
51         int value = itr->second->value;
52         
53         KeyValue tmp(itr->second->key, itr->second->value);
54         keyToNodeItr.erase(itr->second->key);
55         lru.erase(itr->second);
56         lru.push_front(tmp);
57         keyToNodeItr.insert(std::pair<int, std::list<KeyValue>::iterator>(key, lru.begin()));
58         
59         return value;
60     }
61     return -1;
62 }

View Code

因为链表中存储的为cache的实际空间，因此当需要改变cache的位置时，链表及map都需要改变，开销较大。

版本二

使用std::list维护LRU，链表中存储cache的指针。

Runtime：186ms。

 1 #include <list>
 2 #include <unordered_map>
 3 
 4 struct KeyValue {
 5     KeyValue(int pKey, int pValue):key(pKey), value(pValue) {};
 6     int key;
 7     int value;
 8 };
 9 
10 class LRUCache{
11 public:
12     LRUCache(int capacity):_capacity(capacity) {};
13     
14     int get(int key);
15     void set(int key, int value);
16     
17     ~LRUCache();
18     
19 private:
20     std::unordered_map<int, std::list<KeyValue*>::iterator> keyToNodeItr;
21     std::list<KeyValue*> lru;
22     
23     int _capacity;
24 };
25 
26 LRUCache::~LRUCache() {
27     for (auto itr = lru.begin(); itr != lru.end(); ++itr) {
28         delete *itr;
29     }
30 }
31 
32 void LRUCache::set(int key, int value) {    
33     auto itr = keyToNodeItr.find(key);
34     if (itr != keyToNodeItr.end()) { // set value
35         KeyValue* tmp = *(itr->second);
36         tmp->value = value;
37         lru.erase(itr->second);
38         lru.push_front(tmp);
39         itr->second = lru.begin(); // avoid invalid iterator
40     } else { // insert value
41         if (lru.size() == _capacity) { // pop back lru
42             KeyValue* tmp = *(lru.rbegin());
43             keyToNodeItr.erase(tmp->key);
44             delete tmp;
45             lru.pop_back();
46         }
47         lru.push_front(new KeyValue(key, value));
48         keyToNodeItr.insert(std::pair<int, std::list<KeyValue*>::iterator>(key, lru.begin()));
49     }
50 }
51 
52 int LRUCache::get(int key) {
53     auto itr = keyToNodeItr.find(key);
54     if (itr != keyToNodeItr.end()) {
55         KeyValue* kvPtr = *(itr->second);
56         lru.erase(itr->second);
57         lru.push_front(kvPtr);
58         itr->second = lru.begin();
59         return kvPtr->value;
60     }
61     return -1;
62 }

View Code

需要注意的问题是map中存储的为list的迭代器，因此map中仍需要重新设置key到迭代器的映射，避免迭代器失效。

版本三

似乎std::list太笨重了，so实现轻量级双链表代替std::list。

Runtime: 77ms。

  1 struct BiListNode {
  2     BiListNode() {};
  3     BiListNode(int key, int value):key(key), value(value) {};
  4     int key;
  5     int value;
  6     BiListNode* pre;
  7     BiListNode* next;
  8 };
  9 
 10 class BiList {
 11 public:
 12     BiList():_count(0) {
 13         _head = new BiListNode();
 14         _head->pre = _head;
 15         _head->next = _head;
 16     }
 17 
 18     void push_front(BiListNode* pNode);
 19 
 20     void move_front(BiListNode* pNode);
 21     
 22     BiListNode* begin() {
 23         return _head->next;
 24     }
 25     
 26     BiListNode* rbegin() {
 27         return _head->pre;
 28     }
 29     
 30     void pop_back();
 31 
 32     int size() { return _count; }
 33 
 34     ~BiList();
 35 
 36 private:
 37     BiListNode* _head;
 38     int _count;
 39 };
 40 
 41 void BiList::push_front(BiListNode* pNode) {
 42     pNode->next = _head->next;
 43     pNode->pre = _head;
 44     _head->next->pre = pNode;
 45     _head->next = pNode;
 46     if (_head->pre == _head) {
 47         _head->pre = pNode;
 48     }
 49     ++_count;
 50 }
 51 
 52 void BiList::move_front(BiListNode* pNode) {
 53     if (pNode == _head->next) {
 54         return;
 55     }
 56     pNode->pre->next = pNode->next;
 57     pNode->next->pre = pNode->pre;
 58     
 59     pNode->next = _head->next;
 60     pNode->pre = _head;
 61     
 62     _head->next->pre = pNode;
 63     
 64     _head->next = pNode;
 65 }
 66 
 67 void BiList::pop_back() {
 68     BiListNode* tailPtr = _head->pre;
 69     tailPtr->pre->next = _head;
 70     _head->pre = tailPtr->pre;
 71     delete tailPtr;
 72     --_count;
 73 }
 74 
 75 BiList::~BiList() {
 76     for (BiListNode* itr = _head->next; itr != _head; itr = itr->next) {
 77         delete itr;
 78     }
 79     delete _head;
 80 }
 81 
 82 
 83 class LRUCache {
 84 public:
 85     LRUCache(int capacity):_capacity(capacity) {};
 86     
 87     int get(int key);
 88     
 89     void set(int key, int value);
 90     
 91 private:
 92     int _capacity;
 93     
 94     BiList biList;
 95     std::unordered_map<int, BiListNode*> keyToNodePtr;
 96 };
 97 
 98 int LRUCache::get(int key) {
 99     auto itr = keyToNodePtr.find(key);
100     if (itr != keyToNodePtr.end()) {
101         biList.move_front(itr->second);
102         return itr->second->value;
103     }
104     return -1;
105 }
106 
107 void LRUCache::set(int key, int value) {
108     auto itr = keyToNodePtr.find(key);
109     if (itr != keyToNodePtr.end()) { // set value
110         itr->second->value = value;
111         biList.move_front(itr->second);
112     } else { // insert
113         if (biList.size() == _capacity) {
114             keyToNodePtr.erase((biList.rbegin())->key);
115             biList.pop_back();
116         }
117         biList.push_front(new BiListNode(key, value));
118         keyToNodePtr.insert(std::pair<int, BiListNode*>(key, biList.begin()));
119     }
120 }

View Code

自己实现的双链表仅有80行代码，代码运行效率大大提高。

版本四

双链表都自己实现了，就死磕到底，再自己实现个开链哈希表吧。

Runtime：66ms。

  1 struct BiListNode {
  2     BiListNode() {};
  3     BiListNode(int key, int value):key(key), value(value) {};
  4     int key;
  5     int value;
  6     BiListNode* pre;
  7     BiListNode* next;
  8 };
  9 
 10 class BiList {
 11 public:
 12     BiList():_count(0) {
 13         _head = new BiListNode();
 14         _head->pre = _head;
 15         _head->next = _head;
 16     }
 17 
 18     void push_front(BiListNode* pNode);
 19 
 20     void move_front(BiListNode* pNode);
 21     
 22     BiListNode* begin() {
 23         return _head->next;
 24     }
 25     
 26     BiListNode* rbegin() {
 27         return _head->pre;
 28     }
 29     
 30     void pop_back();
 31 
 32     int size() { return _count; }
 33 
 34     ~BiList();
 35 
 36 private:
 37     BiListNode* _head;
 38     int _count;
 39 };
 40 
 41 void BiList::push_front(BiListNode* pNode) {
 42     pNode->next = _head->next;
 43     pNode->pre = _head;
 44     _head->next->pre = pNode;
 45     _head->next = pNode;
 46     if (_head->pre == _head) {
 47         _head->pre = pNode;
 48     }
 49     ++_count;
 50 }
 51 
 52 void BiList::move_front(BiListNode* pNode) {
 53     if (pNode == _head->next) {
 54         return;
 55     }
 56     pNode->pre->next = pNode->next;
 57     pNode->next->pre = pNode->pre;
 58     
 59     pNode->next = _head->next;
 60     pNode->pre = _head;
 61     
 62     _head->next->pre = pNode;
 63     
 64     _head->next = pNode;
 65 }
 66 
 67 void BiList::pop_back() {
 68     BiListNode* tailPtr = _head->pre;
 69     tailPtr->pre->next = _head;
 70     _head->pre = tailPtr->pre;
 71     delete tailPtr;
 72     --_count;
 73 }
 74 
 75 BiList::~BiList() {
 76     for (BiListNode* itr = _head->next; itr != _head; itr = itr->next) {
 77         delete itr;
 78     }
 79     delete _head;
 80 }
 81 
 82 struct hashNode {
 83     hashNode(int key, BiListNode* ptr):key(key), ptr(ptr), next(NULL) {};
 84     int key;
 85     BiListNode* ptr;
 86     hashNode* next;
 87 };
 88 
 89 class HashTable {
 90 public:
 91     HashTable(int capacity);
 92     
 93     hashNode* find(int key);
 94     
 95     void insert(int key, BiListNode* ptr);
 96     
 97     void erase(int key);
 98     
 99     ~HashTable();
100     
101 private:
102     int _capacity;
103     hashNode** hashArray;
104 };
105 
106 HashTable::HashTable(int capacity):_capacity(capacity) {
107     hashArray = new hashNode*[capacity];
108     for (int i = 0; i < _capacity; ++i) {
109         hashArray[i] = NULL;
110     }
111 }
112 
113 hashNode* HashTable::find(int key) {
114     for (hashNode* itr = hashArray[key % _capacity]; itr != NULL;
115             itr = itr->next) {
116         if (itr->key == key) {
117             return itr;
118         }
119     }
120     return NULL;
121 }
122 
123 void HashTable::insert(int key, BiListNode* ptr) {
124     hashNode* tmp = new hashNode(key, ptr);
125     
126     int relativeKey = key % _capacity;
127     
128     if (hashArray[relativeKey] == NULL) {
129         hashArray[relativeKey] = tmp;
130         return;
131     }
132 
133     tmp->next = hashArray[relativeKey];
134     hashArray[relativeKey] = tmp;
135 }
136 
137 void HashTable::erase(int key) {
138     for (hashNode* pre = hashArray[key % _capacity], *itr = pre;
139             itr != NULL; pre = itr, itr = itr->next) {
140         if (itr->key == key) {
141             if (itr != pre)
142                 pre->next = itr->next;
143             else // head
144                 hashArray[key % _capacity] = itr->next;
145 
146             delete itr;
147         }
148     }
149 }
150 
151 HashTable::~HashTable() {
152     for (int i = 0; i < _capacity; ++i) {
153         for (hashNode* itr = hashArray[i]; itr != NULL;) {
154             hashNode* tmp = itr;
155             itr = itr->next;
156             delete tmp;
157         }
158     }
159     delete [] hashArray;
160 }
161 
162 class LRUCache {
163 public:
164     LRUCache(int capacity):_capacity(capacity) {
165         hashTable = new HashTable(1024);
166     };
167     
168     int get(int key);
169     
170     void set(int key, int value);
171     
172     ~LRUCache() { delete hashTable; }
173     
174 private:
175     int _capacity;
176     BiList bilist;
177     HashTable* hashTable;
178 };
179 
180 int LRUCache::get(int key) {
181     hashNode* tmp = hashTable->find(key);
182     if (tmp != NULL) {
183         bilist.move_front(tmp->ptr);
184         return tmp->ptr->value;
185     }
186     return -1;
187 }
188 
189 void LRUCache::set(int key, int value) {
190     hashNode* tmp = hashTable->find(key);
191     if (tmp != NULL) { // set
192         bilist.move_front(tmp->ptr);
193         tmp->ptr->value = value;
194         return;
195     }
196     
197     // insert
198     if (bilist.size() == _capacity) {
199         hashTable->erase((bilist.rbegin())->key);
200         bilist.pop_back();
201     }
202 
203     bilist.push_front(new BiListNode(key, value));
204     hashTable->insert(key, bilist.begin());
205 }

View Code

开链哈希表72行代码

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原文地址：https://www.cnblogs.com/hzhesi/p/4262435.html