LRU Cache -- LeetCode

浏览数：25 / 时间：2015年06月20日

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.

题目描述如上，就是设计一个数据结构，实现LRU算法，支持两种操作，get（查询key对应的value),和set(将key,value加入集合中)

首先，我用双向链表实现，代码细节需要注意的地方较多，因为是处理指针。

  1 class LRUCache{
  2 public:
  3     class Node {
  4     public:
  5         int key, value;
  6         Node *prv, *nxt;
  7         Node():key(0), value(0), prv(NULL), nxt(NULL) {}
  8         Node(int key, int val):key(key), value(val), prv(NULL), nxt(NULL) {}
  9     };
 10     class DoubleLinkedList {
 11     public:
 12         Node *head, *tail;
 13         int size, capacity;
 14         DoubleLinkedList():size(0), capacity(0), head(NULL), tail(NULL) {}
 15         DoubleLinkedList(int c):size(0), capacity(c), head(NULL), tail(NULL) {}
 16         /*~DoubleLinkedList() {
 17             while (head != NULL) {
 18                 Node *tmp = head;
 19                 head = head->nxt;
 20                 delete tmp;
 21             }
 22         }*/
 23         void erase(Node *it) {
 24             if (size == 0) return ;
 25             if (it->prv != NULL && it->nxt != NULL) {
 26                 it->prv->nxt = it->nxt;
 27                 it->nxt->prv = it->prv;
 28             }
 29             if (it->prv == NULL) {
 30                 head = it->nxt;
 31                 if (head != NULL)
 32                     head->prv = NULL;
 33             }
 34             if (it->nxt == NULL) {
 35                 tail = it->prv;
 36                 if (tail != NULL)
 37                     tail->nxt = NULL;
 38             }
 39             size--;
 40         }
 41         int insert(Node *it) {
 42             if (tail != NULL) {
 43                 tail->nxt = it;
 44                 it->nxt = NULL;
 45                 it->prv = tail;
 46                 tail = it;
 47             } else {
 48                 head = tail = it;
 49             }
 50             //if (head == NULL) head = it;
 51             //if (tail == NULL) tail = it;
 52             size++;
 53             if (size > capacity && size > 0) {
 54                 int key = head->key;
 55                 if (size == 1) {
 56                     delete head;
 57                     head = tail = NULL;
 58                 } else {
 59                     Node *tmp = head;
 60                     head = head->nxt;
 61                     head->prv = NULL;
 62                     delete tmp;
 63                     tmp = NULL;
 64                 }
 65                 size--;
 66                 return  key;
 67             }
 68             return -1;
 69         }
 70     };
 71     LRUCache(int capacity) {
 72         //DoubleLinkedList DLL(capacity);
 73         DLL.capacity = capacity;
 74     }
 75     
 76     int get(int key) {
 77         if (HashMap.find(key) == HashMap.end()) {
 78             return -1;
 79         } else {
 80             Node *it = HashMap[key];
 81             DLL.erase(it);
 82             DLL.insert(it);
 83             return (it->value);
 84         }
 85     }
 86     
 87     void set(int key, int value) {
 88         if (HashMap.find(key) != HashMap.end()) {
 89             Node *it = HashMap[key];
 90             it->value = value;
 91             DLL.erase(it);
 92             DLL.insert(it);
 93         } else {
 94             Node *tmp = new Node(key, value);
 95             HashMap.insert(pair<int, Node*>(key, tmp));
 96             int k = DLL.insert(tmp);
 97             if (k != -1) HashMap.erase(k);
 98         }
 99     }
100     
101 private:
102     unordered_map<int, Node*> HashMap;
103     DoubleLinkedList DLL;
104 };

View Code

后来，有人说可以用list来简化代码，我原以为面试的时候应该尽量少的使用STL等高级手段，后来巨巨们说并不是这样，于是就用list实现了一遍。

 1 class LRUCache{
 2 public:
 3     LRUCache(int c) {
 4         capacity = c;
 5     }
 6     
 7     int get(int key) {
 8         int ans = -1;
 9         auto it = disc.find(key);
10         if (it != disc.end()) {
11             ans = it->second->second;
12             data.erase(it->second);
13             data.push_front(pair<int, int>(key, ans));
14             disc[key] = data.begin();
15         }
16         return ans;
17     }
18     
19     void set(int key, int value) {
20         auto it = disc.find(key);
21         if (it != disc.end()) {
22             data.erase(it->second);
23             data.push_front(pair<int, int>(key, value));
24             disc[key] = data.begin();
25         } else {
26             data.push_front(pair<int, int>(key, value));
27             disc[key] = data.begin();
28             if (data.size() > capacity) {
29                 auto it2 = data.end(); it2--;
30                 int k = it2->first;
31                 disc.erase(k);
32                 data.pop_back();
33             }
34         }
35     }
36 private:
37     unordered_map<int, list<pair<int, int> >::iterator > disc;
38     list<pair<int, int> > data; 
39     int capacity;
40 };