LintCode-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->9->null, 14->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 table: [null, 9, null, null, null, 21, 14, null]

 

Given the original hash table, return the new hash table after rehashing .

Note

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

In 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.

In Python, you can directly use -1 % 3, you will get 2 automatically.

Example

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

Solution:

/**
 * 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 size = hashTable.length;
        if (size==0) return null;
        int newSize = 2*size;
        ListNode[] newTable = new ListNode[newSize];
        Arrays.fill(newTable,null);

        for (int i=0;i<size;i++){
            ListNode cur = hashTable[i];
            while (cur!=null){
                ListNode temp = cur;
                cur = cur.next;
                //Calculate the new position for temp.
                int val = (temp.val % newSize + newSize) % newSize;
                if (newTable[val]==null){
                    newTable[val]=temp;
                    temp.next = null;
                } else {
                    ListNode p = newTable[val];
                    while (p.next!=null) p = p.next;
                    p.next = temp;
                    temp.next = null;
                }
            }
        }

        return newTable;
    }
};