Rehashing – Lintcode

http://blog.welkinlan.com/2015/08/21/rehashing-lintcode-java/

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

1 2 3 4 5

[null, 21, 14, null]        ↓    ↓        9   null        ↓       null

The hash function is:

1 2 3

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

1 2	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 .

Example

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

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

Brute-force rehashing

    public ListNode[] rehashing(ListNode[] hashTable) {

        // write your code here

        if (hashTable == null || hashTable.length == 0) {

            return hashTable;

        }

        int size = hashTable.length * 2;

        ListNode[] newTable = new ListNode[size];

        for (int i = 0; i < hashTable.length; i++) {

            ListNode curNode = hashTable[i];

            while (curNode != null) {

                int val = curNode.val;

                if (val >= 0) {

                    insert(newTable, val % size, val);

                } else {

                    insert(newTable, (val % size + size) % size, val);

                }

                curNode = curNode.next;

            }

        }

        return newTable;

    }

    

    private void insert(ListNode[] newTable, int i, int v) {

        if (newTable[i] == null) {

            newTable[i] = new ListNode(v);

        } else {

            ListNode head = newTable[i];

            while (head.next != null) {

                head = head.next;

            }

            head.next = new ListNode(v);

        }

    }

http://www.jiuzhang.com/solutions/rehashing/

    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;
                if (newTable[newindex] == null) {
                    newTable[newindex] = new ListNode(hashTable[i].val);
                   // newTable[newindex].next = null;
                } 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;
    }