【LeetCode with Python】 LRU Cache

博客域名：http://www.xnerv.wang
原题页面：https://oj.leetcode.com/problems/lru-cache/
题目类型：数据结构
难度评价：★★
文本地址：http://blog.csdn.net/nerv3x3/article/details/2920168

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

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.

实现一个LRU缓存。最近最少使用LRU表的定义就不多说了，需要实现的get方法是根据key取value的，因此显然需要一个map，在Python里叫关联数组的帮助。虽然在leetcode中这道题被标为Hard级别，但其实这只是一道工程性质的题目。

class LRUCache:    class LRUNode:        def __init__(self, key, value):            self.prev = None            self.next = None            self.key = key            self.value = value    # @param capacity, an integer    def __init__(self, capacity):        self.map = { }        self.list_head = LRUCache.LRUNode(-1, -1)        self.list_tail = LRUCache.LRUNode(-1, -1)        self.list_head.next = self.list_tail        self.list_tail.prev = self.list_head        self.capacity = capacity    def remove_node(self, node):        node.prev.next = node.next        node.next.prev = node.prev        def append_node(self, new_node):        last_node = self.list_tail.prev        last_node.next = new_node        new_node.prev = last_node        new_node.next = self.list_tail        self.list_tail.prev = new_node    # @return an integer    def get(self, key):        if self.map.has_key(key):            cur = self.map.get(key)            self.remove_node(cur)            self.append_node(cur)            return cur.value        else:            return -1    # @param key, an integer    # @param value, an integer    # @return nothing    def set(self, key, value):        if self.map.has_key(key):            old_node = self.map.get(key)            self.remove_node(old_node)            old_node.value = value            self.append_node(old_node)        else:            if len(self.map) >= self.capacity:                del_node = self.list_head.next                self.remove_node(del_node)                del self.map[del_node.key]            new_node = LRUCache.LRUNode(key, value)            self.append_node(new_node)            self.map[key] = new_node