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 .

Notice

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.

Have you met this question in a real interview?

Yes

Example

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

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

 /**
  * 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) {
         if (hashTable == null || hashTable.length == ) return hashTable;

         ListNode[] newHT = new ListNode[hashTable.length * ];

         for (int i = ; i < hashTable.length; i++) {
             ListNode current = hashTable[i];
             while (current != null) {
                 ListNode next = current.next;
                 current.next = null;
                 add(newHT, current);
                 current = next;
             }
         }
         return newHT;
     }

     public void add(ListNode[] hashTable, ListNode node) {
         int index = node.val % hashTable.length;
         if (index < ) {
             index = hashTable.length + index;
         }
         if (hashTable[index] == null) {
             hashTable[index] = node;
         } else {
             ListNode current = hashTable[index];
             while (current.next != null) {
                 current = current.next;
             }
             current.next = node;
         }
     }
 };