O(1) python solution, 60 lines. using doubly linked list and OrderedDict.

  • 0
    class Freqnode:
        def __init__(self, count):
            self.count = count
            self.next = None
            self.prev = None
            self.kids = collections.OrderedDict()
        def link(self, nextnode):
            self.next = nextnode
            nextnode.prev = self
    class LFUCache(object):
        def __init__(self, capacity):
            self.dict_key_to_freqnode = {}
            self.cap = capacity
            self.head = Freqnode(0)
            self.head.link(Freqnode(0)) # link with a dummy tail
        def get(self, key):
            node = self.dict_key_to_freqnode.get(key)
            if node:
                value = node.kids[key]
                self._updateKey(key, value)
                return value
            return -1
        def _updateKey(self, key, value):
            oldfnode = self.dict_key_to_freqnode[key]
            newfnode = self._getNextFreqnode(oldfnode, key, value)
            del oldfnode.kids[key]
        def set(self, key, value):
            if self.cap == 0: return
            if len(self.dict_key_to_freqnode) == self.cap and key not in self.dict_key_to_freqnode:
                # reaching cap, need to kick out least recently used key with the least frequent use count.
                node = self.head.next
                lru_key = node.kids.popitem(False)
                del self.dict_key_to_freqnode[lru_key[0]]
            if key in self.dict_key_to_freqnode:
                self._updateKey(key, value)
                self._getNextFreqnode(self.head, key, value)
        def _removeFreqnodeIfEmpty(self, fnode):
            if len(fnode.kids) == 0:
        def _getNextFreqnode(self, fnode, key, value):
            # create a new node if the +1 count doesn't exist
            if fnode.next.count != fnode.count + 1:
                node = Freqnode(fnode.count+1)
            self.dict_key_to_freqnode[key] = fnode.next
            fnode.next.kids[key] = value
            return fnode.next

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