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 number, 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 .

Example

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

 
/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @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
     */    
    vector<ListNode*> rehashing(vector<ListNode*> hashTable) {
       int n = hashTable.size();
       int nn = 2*n;
       vector<ListNode*> headTable(nn);   
       vector<ListNode*> tailTable(nn);   
       
       for(int i = 0; i < n; i++)
       {
           ListNode *list = hashTable[i];
           while(list != NULL)
           {
               int indx = list->val%nn;
               
               // important, list->val can be negative
               if (indx < 0) indx += nn;
               
               if (tailTable[indx])
                   tailTable[indx]->next = list;
               tailTable[indx] = list;
               
               if (!headTable[indx]) headTable[indx] = list;
               
               ListNode *t = list;
               list = t->next;
               t->next = NULL;
           }
       }
       
       return headTable;
    }
};