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]

这道题就是根据条件老老实实的做

 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         // write your code here

19         int oldSize = hashTable.length;

20         int newSize = oldSize * 2;

21         if (hashTable==null || oldSize==0) return null;

22         ListNode[] res = new ListNode[newSize];

23         for (int i=0; i<oldSize; i++) {

24             if (hashTable[i] != null) rehash(hashTable, res, i);

25         }

26         return res;

27     }

28     

29     public void rehash(ListNode[] hashTable, ListNode[] res, int i) {

30         int newSize = res.length;

31         ListNode cur = hashTable[i];

32         while (cur != null) {

33             int val = cur.val;

34             int newPos = val>=0? val%newSize : (val%newSize+newSize)%newSize;

35             if (res[newPos] == null) res[newPos] = new ListNode(val);

36             else {

37                 ListNode temp = res[newPos];

38                 while (temp.next != null) {

39                     temp = temp.next;

40                 }

41                 temp.next = new ListNode(val);

42             }

43             cur = cur.next;

44         }

45     }

46 };