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:
1 /** 2 * Definition for ListNode 3 * public class ListNode { 4 * int val; 5 * ListNode next; 6 * ListNode(int x) { 7 * val = x; 8 * next = null; 9 * } 10 * } 11 */ 12 public class Solution { 13 /** 14 * @param hashTable: A list of The first node of linked list 15 * @return: A list of The first node of linked list which have twice size 16 */ 17 public ListNode[] rehashing(ListNode[] hashTable) { 18 int size = hashTable.length; 19 if (size==0) return null; 20 int newSize = 2*size; 21 ListNode[] newTable = new ListNode[newSize]; 22 Arrays.fill(newTable,null); 23 24 for (int i=0;i<size;i++){ 25 ListNode cur = hashTable[i]; 26 while (cur!=null){ 27 ListNode temp = cur; 28 cur = cur.next; 29 //Calculate the new position for temp. 30 int val = (temp.val % newSize + newSize) % newSize; 31 if (newTable[val]==null){ 32 newTable[val]=temp; 33 temp.next = null; 34 } else { 35 ListNode p = newTable[val]; 36 while (p.next!=null) p = p.next; 37 p.next = temp; 38 temp.next = null; 39 } 40 } 41 } 42 43 return newTable; 44 } 45 };