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

<code>[null, 21, 14, null]
       ↓    ↓
       9   null
       ↓
      null
</code>

The hash function is:

<code>int hashcode(int key, int capacity) {
    return key % capacity;
}
</code>

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

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

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]

Note

For negative integer in hash table, the position can be calculated as follow:

  • C++/Java: if you directly calculate -4 % 3 you will get -1. You can use function: a % b = (a % b + b) % b to make it is a non negative integer.
  • Python: you can directly use -1 % 3, you will get 2 automatically.

Solution:

Note: For negative integer in hash table, the position can be calculated as follow:
if you directly calculate -4 % 3 you will get -1.
You can use function: a % b = (a % b + b) % b to make it is a non negative integer.

public ListNode[] rehashing(ListNode[] hashTable) {
    if (hashTable == null) {
        return null;
    }
    ListNode[] newHash = new ListNode[hashTable.length * 2];
    int capacity = newHash.length;
    for (int i = 0; i < hashTable.length; i++) {
        ListNode node = hashTable[i];
        while (node != null) {
            int index = mod(node.val, capacity);
            ListNode next = node.next; //need to remove the next pointer
            node.next = null;
            //if it is the first one
            if (newHash[index] == null) {
                newHash[index] = node;
            } else {
                ListNode runner = newHash[index];
                while (runner.next != null) {
                    runner = runner.next;
                }
                runner.next = node;
            }
            hashTable[i] = next; //set to its next node
            node = next;
        }
    }
    return newHash;
}

//For negative integer in hash table, the position can be calculated as follow:
//if you directly calculate -4 % 3 you will get -1.
//You can use function: a % b = (a % b + b) % b to make it is a non negative integer.
public int mod(int A, int module) {
    return (A % module + module) % module;
}
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