460.md

460. LFU Cache

Solution 1

class Node:
    def __init__(self, key=0, value=0):
        self.key = key
        self.value = value
        self.freq = 1
        self.prev = None
        self.next = None

class LFUCache(object):

    def __init__(self, capacity):
        """
        :type capacity: int
        """
        self.capacity = capacity
        self.key_to_node = {}
        def linked_list():
            dummy = Node()
            dummy.prev = dummy
            dummy.next = dummy
            return dummy
        self.freq_to_dummy = collections.defaultdict(linked_list)

    def get_node(self, key):
        if key not in self.key_to_node:
            return None
        node = self.key_to_node[key]
        self.remove(node)
        dummy = self.freq_to_dummy[node.freq]
        if dummy.prev == dummy:
            del self.freq_to_dummy[node.freq]
            if self.min_freq == node.freq:
                self.min_freq += 1
        node.freq += 1
        self.push_front(self.freq_to_dummy[node.freq], node)
        return node
    
    def remove(self, node):
        node.prev.next = node.next
        node.next.prev = node.prev
    
    def push_front(self, dummy, node):
        node.prev = dummy
        node.next = dummy.next
        node.prev.next = node
        node.next.prev = node
    
    def get(self, key):
        """
        :type key: int
        :rtype: int
        """
        node = self.get_node(key)
        return node.value if node else -1

    def put(self, key, value):
        """
        :type key: int
        :type value: int
        :rtype: None
        """
        node = self.get_node(key)
        if node:
            node.value = value
            return
        if len(self.key_to_node) == self.capacity:
            dummy = self.freq_to_dummy[self.min_freq]
            back_node = dummy.prev
            del self.key_to_node[back_node.key]
            self.remove(back_node)
            if dummy.prev == dummy:
                del self.freq_to_dummy[self.min_freq]
        new_node = Node(key, value)
        self.key_to_node[key] = new_node
        self.push_front(self.freq_to_dummy[1], new_node)
        self.min_freq = 1

# Your LFUCache object will be instantiated and called as such:
# obj = LFUCache(capacity)
# param_1 = obj.get(key)
# obj.put(key,value)