LRU

LRU: 缓存置换算法，mysql page, redis缓存等使用
实现一个LRU, 主要需要考虑几点:
一个双向链表，一个hash map

#
# @lc app=leetcode.cn id=146 lang=python
#
# [146] LRU缓存机制
#

# @lc code=start
class DLink(object):

    def __init__(self, key=None, value=None, prev=None, next=None):
        self.key = key
        self.value = value
        self.prev = prev
        self.next = next


class LRUCache(object):

    def __init__(self, capacity):
        """
        :type capacity: int
        """
        self.capacity = capacity
        self.head = DLink()
        self.tail = DLink()
        self.head.prev = self.tail
        self.tail.next = self.head
        self.page_cache = {}
        self.current_volumn = 0

    def get(self, key):
        """
        :type key: int
        :rtype: int
        """
        result = self.page_cache.get(key)
        if result:
            self.add_head(self.result)
            return result.value
        else:
            return -1

    def put(self, key, value):
        """
        :type key: int
        :type value: int
        :rtype: None
        """
        node = self.page_cache.get(key)
        if node:
           # 这里需要注意有node的时候，需要把node前置
            node.value = value
            self.remove_node(node)
            self.add_head(node)
            return 

        if self.current_volumn < self.capacity:
            node = DLink(key, value, self.tail)
            self.add_tail(node)
            self.current_volumn += 1
            self.page_cache[key] = node

        else:
            node = DLink(key, value, None, self.head)
            self.add_head(node)
            t = self.remove_tail()
            self.page_cache.pop(t.key)
            self.page_cache[key] = value

    def remove_node(node):
        node.prev.next = node.next
        node.next.prev = node.prev

    def add_head(node):
        node.prev = self.head
        node.next = self.head.next
        self.head.next = node
        self.head.next.prev = node

    def add_tail(node):
        self.remove_node(node)
        self.add_head(node)

    def remove_tail():
        node = self.tail.prev
        self.remove_node(node)
        return node