[LeetCode] 146. LRU Cache_Hard tag: Hash, Linked List

﹏ヽ暗。殇╰゛Y 2021-11-16 07:20 205阅读 0赞

Design and implement a data structure for [Least Recently Used (LRU) cache][Least Recently Used _LRU_ cache]. It should support the following operations: `get` and `put`.

`get(key)` - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.  
`put(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.

Follow up:  
Could you do both operations in O(1) time complexity?

Example:

LRUCache cache = new LRUCache( 2 /* capacity */ );
    
    cache.put(1, 1);
    cache.put(2, 2);
    cache.get(1);       // returns 1
    cache.put(3, 3);    // evicts key 2
    cache.get(2);       // returns -1 (not found)
    cache.put(4, 4);    // evicts key 1
    cache.get(1);       // returns -1 (not found)
    cache.get(3);       // returns 3
    cache.get(4);       // returns 4

这个题目实际上就是用linked list去记录cache中存在的点，因为delete和move，add都是O(1) 的复杂度，然后用hash 去存cache中现在有的数。

Note：因为存的是key，所以linked list里面还多一个key的值。另外由于如果用单向的linked list，我们要知道它的pre node，所以在hash中我们存的实际上是该node的pre node，因此也需要有个dummy node。另外因为有去掉head，add to tail的操作，所以可以另写两个function，有助于debug，也使程序看起来更加有条理。

Code:

class ListNode:
        def __init__(self, key, value):
            self.key = key
            self.val = value
            self.next = None
    
    class LRUCache:
        def __init__(self, capacity):
            self.capacity = capacity
            self.d = dict()  # hash
            self.dummy = ListNode(0)
            self.tail = self.dummy
        def popHead(self):
            head = self.dummy.next
            self.dummy.next = head.next
            self.d.pop(head.key)
            if head.next:
                self.d[head.next.key] = self.dummy
        def addToTail(self, node):
            self.tail.next = node
            self.d[node.key] = self.tail
            self.tail = node
        def moveToTail(self, pre):
            if pre.next == self.tail:
                return 
            node = pre.next
            pre.next = node.next
            if node.next:
                self.d[node.next.key] = pre
                node.next = None
            self.addToTail(node)
        def get(self, key):
            if key in self.d:
                self.moveToTail(self.d[key])
                return self.d[key].next.val
            return -1
        def put(self, key, value):
            if key in self.d:
                self.moveToTail(self.d[key])
                self.d[key].next.val = value
            else:
                self.addToTail(ListNode(key, value))
                if len(self.d) > self.capacity:
                    self.popHead()

转载于:https://www.cnblogs.com/Johnsonxiong/p/10885036.html

[Least Recently Used _LRU_ cache]: https://en.wikipedia.org/wiki/Cache_replacement_policies#LRU