# 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;
}```