Problem

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 .

Notice

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

Example

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

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

思路

• 注意该题需要问清楚几个问题
• 1. 出现负数怎么办 ?
  2. 当前的数放在链表最前还是最后?

    public ListNode[] rehashing(ListNode[] hashTable) {
        if (hashTable == null || hashTable.length == 0) {
            return null;
        }
        ListNode[] table = new ListNode[2 * hashTable.length];
        int capacity = table.length;
        for (ListNode list: hashTable) {
            while (list != null) {
                ListNode cur = list;
                list = list.next;
                cur.next = null;
                int code = hashcode(cur.val, capacity);
                ListNode newList = table[code];

                while (newList != null && newList.next != null) {
                    newList = newList.next;
                }
                if (newList == null) {
                    table[code] = cur;   
                } else {
                    newList.next = cur;
                }            
            }
        }
        return table;
    }

    private int hashcode(int key, int capacity) {
        if (key >= 0) {
            return key % capacity;
        } else {
            return (key % capacity + capacity) % capacity;
        }
    }