# Rehashing

## Problem

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]

## 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.

## Code - Java

/**
* Definition for ListNode
* public class ListNode {
*   int val;
*   ListNode next;
*   ListNode(int x) {
*     val = x;
*     next = null;
*  }
* }
*/
public 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
*/
public ListNode[] rehashing(ListNode[] hashTable) {
int oldSize = hashTable.length;
int newSize = 2 * oldSize;
ListNode[] result = new ListNode[newSize];

for (int i = 0; i < oldSize; i++) {
ListNode node = hashTable[i];
while (node != null) {
int newHash = hashcode(node.val, newSize);
if (result[newHash] == null) {
result[newHash] = new ListNode(node.val);
} else {
ListNode p = result[newHash];
while (p.next != null) p = p.next;
p.next = new ListNode(node.val);
}
node = node.next;
}
}
return result;
}

private int hashcode(int key, int capacity) {
if (key >= 0) {
return key % capacity;
} else {
return (key % capacity + capacity) % capacity;
}
}
};