Rehashing

The size of the hash table is not determinate at the very beginning. If the total size of keys is too large (e.g. size >= capacity / 10), we should double the size of the hash table and rehash every keys. Say you have a hash table looks like below:

size=3, capacity=4

[null, 21, 14, null]
       ↓    ↓
       9   null
       ↓
      null

The hash function is:

int hashcode(int key, int capacity) {
    return key % capacity;
}

here we have three numbers, 9, 14 and 21, where 21 and 9 share the same position as they all have the same hashcode 1 (21 % 4 = 9 % 4 = 1). We store them in the hash table by linked list.

rehashing this hash table, double the capacity, you will get:

size=3, capacity=8

index:   0    1    2    3     4    5    6   7
hash : [null, 9, null, null, null, 21, 14, null]

Given the original hash table, return the new hash table after rehashing .

Example:

Given [null, 21->9->null, 14->null, null],

return [null, 9->null, null, null, null, 21->null, 14->null, null]

Solution:

the hastable is passed in we can do double it direclty

进来后就直接double, 不用考虑是否要double的问题。

数组代替hash表，所以是数组操作。

"""
Definition of ListNode
class ListNode(object):

    def __init__(self, val, next=None):
        self.val = val
        self.next = next
"""
class Solution:
    """
    @param hashTable: A list of The first node of linked list
    @return: A list of The first node of linked list which have twice size
    """
    def rehashing(self, hashTable):
        # write your code here
        newHashTable = [None for i in range(len(hashTable) * 2)]
        for item in hashTable:
            node = item
            while node:
                val = node.val
                self.addNewNode(newHashTable, val)
                node = node.next
        return newHashTable

    def addNewNode(self, newHashTable, val):
        key = val % len(newHashTable)
        if newHashTable[key]:
            self.addListNode(newHashTable[key], val)
        else:
            newHashTable[key] = ListNode(val)


    def addListNode(self,node, val):
        if node.next:
            self.addListNode(node.next, val)
        else:
            node.next = ListNode(val)