# 实现LRU

```python
'''
方法一
class LRUCache:
    #@param capacity,an integer
    def __init__(self,capacity):
        self.cache ={}
        self.used_list=[]
        self.capacity = capacity
    #@return an integer
    def get(self,key):
        if key in self.cache:
            #使用一个list来记录访问的顺序，最先访问的放在list的前面，最后访问的放在list的后面，故cache已满时，则删除list[0]，然后插入新项；
            if key != self.used_list[-1]:
                self.used_list.remove(key)
                self.used_list.append(key)
            return self.cache[key]
        else:
            return -1
    def set(self,key,value):
        if key in self.cache:
            self.used_list.remove(key)
        elif len(self.cache) == self.capacity:
            self.cache.pop(self.used_list.pop(0))
        self.used_list.append(key)
        self.cache[key] = value
'''

#方法二：
import collections

#基于orderedDict实现
class LRUCache(collections.OrderedDict):
    '''
    function:利用collection.OrdereDict数据类型实现最近最少使用的算法
    OrdereDict有个特殊的方法popitem(Last=False)时则实现队列，弹出最先插入的元素
    而当Last=True则实现堆栈方法，弹出的是最近插入的那个元素。
    实现了两个方法：get(key)取出键中对应的值，若没有返回None
    set(key,value)更具LRU特性添加元素
    '''
    def __init__(self,size=5):
        self.size = size
        self.cache = collections.OrderedDict()#有序字典

    def get(self,key):
        if key in self.cache.keys():
            #因为在访问的同时还要记录访问的次数（顺序）
            value = self.cache.pop(key)
            #保证最近访问的永远在list的最后面
            self.cache[key] = value
            return value
        else:
            value = None
            return value

    def set(self,key,value):
        if key in self.cache.keys():# 存在就移除，然后插入新值
            self.cache.pop(key)
            self.cache[key] = value
        elif self.size == len(self.cache):
              # 满了，弹出最老的数据，插入新的
            self.cache.popitem(last=False)
            self.cache[key] = value
        else:
            self.cache[key] = value

if __name__ == '__main__':
    test = LRUCache()
    test.set('a',1)
    test.set('b',2)
    test.set('c',3)
    test.set('d',4)
    test.set('e',5)
    # test.set('f',6)
    print (test.get('a'))
```


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