Data Structure I:heap&hash&stack&queue

problem I: first unique character in a string(analogy to LRU)
idea: solve this problem in a data stream manner !(doubly linked list+hash)
class listnode:
    def __init__(self, val):
        self.prev, self.next, self.val = None, None, val
        
class Solution:
    #linked list maintain a candidate solution which contains all unique element !!!
    def firstUniqChar(self, str):
        # Write your code here
        hashtable, dummy = dict(), listnode(0)
        tail = dummy
        for ch in str:
            if ch not in hashtable.keys():
                hashtable[ch] = listnode(ch)
                tail.next = hashtable[ch]
                hashtable[ch].prev = tail
                tail = hashtable[ch]
            else:
                curr = hashtable[ch]
                if curr.prev != None:  
                #when duplicated element detected, we set.prev be None to remove from the entire list since
                #if node is in the list its prev pointer must not be None ! and retain its hash value to help us check if this is 
                #a duplicated element !!
                    if curr == tail:
                        tail = tail.prev
                    if curr.prev == dummy:
                        dummy.next = curr.next
                        if curr.next != None:
                            curr.next.prev = dummy
                    else:
                        curr.prev.next = curr.next
                        if curr.next != None:
                            curr.next.prev = curr.prev
                    curr.prev = None
                
        return dummy.next.val