0129.Rehashing.java

/*
Description
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 .

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.
0 <= capacity <= 20000<=capacity<=2000
0 <= size <= 100000<=size<=10000

Example
Example 1:

Input:

hashTable = [null, 21->9->null, 14->null, null]
Output:

[null, 9->null, null, null, null, 21->null, 14->null, null]
Explanation:

Double the capacity of the hash table and rearrange all the hash values.
*/

/**
 * 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) {
        // write your code here
        if (hashTable.length <= 0) {
            return hashTable;
        }

        int newcapacity = 2 * hashTable.length;
        ListNode[] newTable = new ListNode[newcapacity];

        for (int i = 0; i < hashTable.length; i++) {
            while (hashTable[i] != null){
                int newindex = (hashTable[i].val % newcapacity + newcapacity) % newcapacity;   //pay attention: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.
                if (newTable[newindex] == null) {
                    newTable[newindex] = new ListNode(hashTable[i].val);
                } else {
                    ListNode dummy = newTable[newindex];
                    while (dummy.next != null) {
                        dummy = dummy.next;
                    }
                    dummy.next = new ListNode(hashTable[i].val);
                }
                hashTable[i] = hashTable[i].next;
            }
        }
        return newTable;
    }
}