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;
        }
    }
};