Rehashing.java

M
1528088505
tags: Hash Table

给一个Hash Table, 是用 linked list 做的. 问题是: capacity太小, collision太多的情况下, 需要double capacity 然后rehash.

#### Hash Table
- 明白hashCode() function的意义: 拿到hashKey的时候, 用hashKey%capacity 来做hash code
- hashcode就是hash map里面的index
- 明白collision handling 的方式, 和如何double capacity而rehashing
- 都是基本操作, 概念实现

```
/*
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 .

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

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

Note
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.
Tags Expand 
LintCode Copyright Hash Table

Thoughts:
1. Loop through the hashtable[] and find longest, calcualte new capacity
2. rehash

*/

/**
 * Definition for ListNode
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
 
public class Solution {
    /**
     * @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
     */    
    public ListNode[] rehashing(ListNode[] hashTable) {
        if (hashTable == null || hashTable.length == 0) {
            return hashTable;
        }

        //Calculate new capacity, double the hashtable size
        int capacity = hashTable.length * 2;

        ListNode[] rst = new ListNode[capacity];
        for (int i = 0; i < hashTable.length; i++) {
            ListNode node = hashTable[i]; // process one hashkey (a linked list)
            while (node != null) {
                ListNode newNode = new ListNode(node.val);
                int hCode = hashcode(newNode.val, capacity);
                if (rst[hCode] == null) {
                    rst[hCode] = newNode;
                } else {
                    ListNode collisionNode = rst[hCode];
                    while (collisionNode.next != null) {
                        collisionNode = collisionNode.next;
                    }
                    collisionNode.next = newNode;
                }
                node = node.next;
            }
        }

        return rst;
    }

    public int hashcode(int key, int capacity) {
        if (key < 0) {
            return (key % capacity + capacity) % capacity;
        } else {
            return key % capacity;
        }
    }
};
```